I don't know anything about PVM, but its website is http://www.epm.ornl.gov/pvm/pvm_home.html
Perhaps someone else can fill in the details, and comparison with MessagePassingInterface, etc.
PVM is slightly lower performance than MPI, but works well in a heterogeneous environment, and is able to directly spawn slave tasks on nodes in the virtual machine. Unlike MPI, PVM does not have asynchronous communication capabilities, only simple synchronous ones. PVM makes use of XDR between unlike nodes (RFC 1014), and native data representations between like nodes. PVM programs are almost always structured as a master and many slaves.
One well known application for PVM is PVMPOV, a patch to POV-Ray ray tracer to allow rendering in parallel over PVM. The povray-3.5 (in non-free) has been been patched.
The same security warning applies to PVM as MPI -- rsh is bad. PVM does support using ssh, and has an alternatives entry for deciding which to use.