Virtual Machine

it is a software simulation of a machine which can perform operation like a physical machine.

There are two types of JVM

hardware or software based VM
application or process based VM

Hardware and system based VM

it provides several logical system on the same computer with strong isolation from each other.i.e. On one physical machine we are defining multiple logical machine.
The main advantage of hardware based virtual is hardware resources sharing and improve utilization of hardware resources.
 
Ex:  KVM (kernal based VM), VMWare, Xen, Cloud computing etc. 

Application or  process based VM This virtual machine  access runtime engine to run  a particular programming language applications.
Ex:   
JVM  (Java Virtual Machine) (access runtime engine to run Java based application)
PVT (Paret Virtual Machine)  (  access runtime engine to run .net based application )