GNU Classpath provides core class libraries for use with virtual machines and compilers for the java programming language. GNU Classpath 1.0 will be fully compatible with the 1.1 and 1.2 API specifications, in addition to having significant compatibility with the 1.3, 1.4, 1.5 and 1.6 APIs. 

 It is the mother of most of the JVMs. Check out the <A HREF="http://www.gnu.org/software/classpath/stories.html#jvm" target="_blank">list</A> of other Virtual Machine products using GNU Classpath as its base. Intel and IBM are using GNU Classpath in their research

GNU Classpath provides core class libraries for use with virtual machines and compilers for the java programming language. GNU Classpath 1.0 will be fully compatible with the 1.1 and 1.2 API specifications, in addition to having significant compatibility with the 1.3, 1.4, 1.5 and 1.6 APIs.

GNU Classpath - Mother of most of the VM

Open JDK is the open source implementation of Java platform. It has loads of sub projects like JDK6, JDK7, HotSpot, Graphics, Swing, Web etc. 
 
 <A HREF="http://openjdk.java.net/projects/jdk6/" target="_blank">JDK 6</A> and <A HREF="http://openjdk.java.net/projects/jdk7/" target="_blank">JDK 7</A> projects are based on the specification of Java SE. OpenJDK 6 sources have passed the Java SE 6 Test Compatibility Kit (TCK) suite. <A HREF="http://icedtea.classpath.org/wiki/Main_Page" target="_blank">IcedTea</A> builds the source from Open JDK and adds 64 bit plugin and Java web start etc. 
 
 Open JDK is actively developed and it is compatible with Java specification.

Open JDK is the open source implementation of Java platform. It has loads of sub projects like JDK6, JDK7, HotSpot, Graphics, Swing, Web etc.

Open JDK - Open Source implementation of Java

Java Standard Edition provides Java development kit and Runtime to develop and deploy Java programs. Developed by Sun and take over by Oracle. It has support for different environments like Desktop, Embedded, Web, Realtime etc. 

 Scripting languages framework extends support for Ruby, Python, and other languages. Full JDBC4 implementation providing improved XML support for Databases. It has built-in support for JNI, Reflection, JAAS, JMX and lot more. There could be any other VM but no one could match its features.

Java Standard Edition provides Java development kit and Runtime to develop and deploy Java programs. Developed by Sun and take over by Oracle. It has support for different environments like Desktop, Embedded, Web, Realtime etc.

Java SE - Creator of Java

GraalVM is a universal virtual machine for running applications written in JavaScript, Python 3, Ruby, R, JVM-based languages like Java, Scala, Kotlin, and LLVM-based languages such as C and C++. GraalVM removes the isolation between programming languages and enables interoperability in a shared runtime. It can run either standalone or in the context of OpenJDK, Node.js, Oracle Database, or MySQL. 


Graal - High-Performance Polyglot VM 

KVM (for Kernel-based Virtual Machine) is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V). It consists of a loadable kernel module, kvm.ko, that provides the core virtualization infrastructure and a processor specific module, kvm-intel.ko or kvm-amd.ko.  Using KVM, one can run multiple virtual machines running unmodified Linux or Windows images. Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc. 

kernel virtual machine

The Xen hypervisor is a layer of software running directly on computer hardware replacing the operating system thereby allowing the computer hardware to run multiple guest operating systems concurrently. It suppors x86, x86-64, Itanium, Power PC, and ARM processors allow the Xen hypervisor to run on a wide variety of computing devices. With Xen virtualization, a thin software layer known as the Xen hypervisor is inserted between the server's hardware and the operating system. This provides an abstraction layer that allows each physical server to run one or more "virtual servers", effectively decoupling the operating system and its applications from the underlying physical server.

The Xen hypervisor is a layer of software running directly on computer hardware replacing the operating system thereby allowing the computer hardware to run multiple guest operating systems concurrently. It suppors x86, x86-64, Itanium, Power PC, and ARM processors allow the Xen hypervisor to run on a wide variety of computing devices.

Xen - Open Source Virtualization

VirtualBox is a powerful x86 and AMD64/Intel64 virtualization product for enterprise as well as home use. VirtualBox runs on Windows, Linux, Macintosh, and Solaris hosts and supports a large number of guest operating systems. So, for example, you can run Windows and Linux on your Mac. VirtualBox is a so-called hosted hypervisor. It requires an existing operating system to be installed. It can thus run alongside existing applications on that host. 

VirtualBox - Cross-Platform Virtualization Application

Kaffe is a clean room implementation of the Java virtual machine, plus the associated class libraries needed to provide a Java runtime environment. It has support for Garbage Collector, Debugging, Profiler, Benchmarking etc. It has been ported to multiple platforms. Among the open source VM, Kaffe is actively developed.

  Kaffe is not an officially licensed version of the Java virtual machine. It contains no Sun source code and was developed without even looking at the Sun source code. Kaffe is a great choice as a base for virtual machine education and research.

Kaffe is a clean room implementation of the Java virtual machine, plus the associated class libraries needed to provide a Java runtime environment. 

Kaffe - Not Java

Apache Harmony is a modular Java runtime with class libraries and associated tools. One of the goals of the Harmony project is to create a design modular enough so that parts of the VM can be implemented in different languages. This project started in 2005 and it is under active development. 

 It adheres to the Java specification. It uses GNU classpath and JCVM a java byte code to C files converter. It has DRLVM, a virtual machine written in C++. It supports JIT compilation, JNI, Garbage Collector and Profile Collector etc.

Apache Harmony is a modular Java runtime with class libraries and associated tools. One of the goals of the Harmony project is to create a design modular enough so that parts of the VM can be implemented in different languages. This project started in 2005 and it is under active development.

Harmony - Open Source Java SE

Proxmox Virtual Environment is a powerful Open Source Server Virtualization Platform, based on KVM and OpenVZ. It gives you near-bare-metal performance and leading scalability for your  workloads. You can virtualize even the most demanding application workloads. It supports 32 bit and 64 bit Windows and Linux servers.

Proxmox - Virtualization Platform

IKVM.NET is an implementation of Java for Mono and the Microsoft .NET Framework. It has JVM implemented in .NET. A .NET implementation of Java class libraries. It is interoperable between Java and .NET. 

 IKVM.NET includes ikvmc, a Java bytecode to .NET IL translator. If you have a Java library that you would like to use in a .NET application, run ikvmc -target:library mylib.jar to create mylib.dll. 		

IKVM provides a way for you to develop .NET applications in Java. Although IKVM.NET does not include a Java compiler for .NET, you can use any Java compiler to compile Java source code to JVM bytecode, then use ikvmc -target:exe myapp.jar to produce a .NET executable. You can even use .NET API's in your Java code using the included ikvmstub application.

IKVM.NET is an implementation of Java for Mono and the Microsoft .NET Framework. It has JVM implemented in .NET. A .NET implementation of Java class libraries. It is interoperable between Java and .NET.

IKVM.NET - Java for Mono and .NET Framework

QEMU is a generic and open source machine emulator and virtualizer. When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performance.
 
When used as a virtualizer, QEMU achieves near native performances by executing the guest code directly on the host CPU. QEMU supports virtualization when executing under the Xen hypervisor or using the KVM kernel module in Linux. When using KVM, QEMU can virtualize x86, server and embedded PowerPC, and S390 guests.

QEMU is a generic and open source machine emulator and virtualizer. When used as a machine emulator, QEMU can run OSes and programs made for one machine (e.g. an ARM board) on a different machine (e.g. your own PC). By using dynamic translation, it achieves very good performance.


QEMU - Machine Emulator and Virtualizer

JC Virtual machine converts Java class files to C source code. It uses <A HREF="http://www.sable.mcgill.ca/soot/" target="_blank">SOOT</A> Java Bytecode analysis framework to convert the Java files to C. It could also compiles with GCC and loads them with built-in ELF loader. Additional to that, JC provides a complete Java runtime with optimization to increase performance. 

 JC optimizes the code at code generation time which increases its runtime performance. It does have good interpreter but most of the jobs are done at the code generation time. When combined with the proven power of GCC's C optimizer the results are impressive. 

 Additional to C conversion and Java runtime, It has thread support based on the POSIX pthread(3). Support for JNI, Reflection and Java 2 style loader support is also available. It uses <A HREF="http://www.gnu.org/software/classpath/" target="_blank">GNU Classpath</A> to provide java runtime.

JC Virtual machine converts Java class files to C source code. It could also compiles with GCC and loads them with built-in ELF loader. Additional to that, JC provides a complete Java runtime with optimization to increase performance.


JC Virtual Machine - Converts Java to C

cacaovm is a Java Virtual Machine (JVM) which uses Just-In-Time (JIT) compilation to execute Java methods natively. cacaovm uses GNU Classpath or OpenJDK or phoneME as Java runtime library. The cacaovm project started as a research JVM to explore new implementation techniques. The first version for the Alpha was released in February 1997 as a binary. 

 CACAO supports i386, x86_64, Alpha, ARM, MIPS 32/64, PowerPC 32/64, S390 architectures. CACAO is almost fully compliant with the Java Virtual Machine Specification. It has a complete byte code verifier that uses subtype constraints to guarantee type-safety without unnecessary loading. A tailored CACAO can be built by enabling or disabling couple of features to reduce executable size.

cacaovm is a Java Virtual Machine (JVM) which uses Just-In-Time (JIT) compilation to execute Java methods natively. cacaovm uses GNU Classpath or OpenJDK or phoneME as Java runtime library. The cacaovm project started as a research JVM to explore new implementation techniques. The first version for the Alpha was released in February 1997 as a binary.

Cacaovm - Just-In-Time (JIT) Compiler

HHVM (aka the HipHop Virtual Machine) is a new open-source virtual machine designed for executing programs written in PHP. HHVM uses a just-in-time compilation approach to achieve superior performance while maintaining the flexibility that PHP developers are accustomed to. 

HHVM - HipHop Virtual Machine for PHP

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