Q.What do you mean by Parallel virtual machine?

PVM is essentially a simulation of a computer machine running parallel programs. It is a software package which allows a heterogeneous collection of UNIX and/or Windows computers hooked together by a network to be used like single large parallel computer. It comprises a combination of good characteristics of different operating systems and architectures. So large computational problems are able to be solved more cost effectively by using combined power as well as memory of different computers. The software is highly portable. The source that is available free through netlib has been compiled on various computing machines from laptops to CRAY machines. 

PVM allows users to use their existing computer hardware to resolve much larger problems at minimal additional cost. Hundreds of sites worldwide are employing PVM to resolve significant industrial, scientific and medical problems. Additional PVM's are being used as an educational tool to teach parallel programming. With millions of users PVM has become de facto standard for distributed computing globally.