Difference between JVM , JRE and JDK.

Internal Architecture of JVM

JVM 

1. It stands for "Java Virtual Machine", which is used for execution of "Java byte code" that is output of compilation of ".java" file by Java compiler(javac ...).                               
2. It interprets Byte code to machine understandable code(Machine Language).    
3. JVM is platform dependent and it is responsible for "Garbage collection","array bound checking" etc.                                   
4. JVM is called Virtual because it provide Machine Interface that does not depend on Operating System and Machine Hardware Architecture.

JRE

Component of  JDK 

1. It stand for "Java Runtime Environment". It contains JVM, class libraries and other supporting files.                                              
2. JRE does not contain any development tool like debugger, compiler etc.                            
3. When JVM runs it uses class libraries and other supporting files provided by JRE.           
4. To run any java program JRE must be installed in the system.

JDK

1. It stands for "Java Development Kit". As the name suggest it contain the tools that will use to develop java programs like Java Compiler(javac.exe), Java Application Launcher(java.exe), Appletviewer etc.         
2. For development of java programs JDK is needed and for running purpose JRE is needed i.e JRE=JVM + Java Packages classes(util, math,awt,swing,language etc) + runtime libraries.