I initially had to choose between PVM and MPI, and I ended up using MPI only because that was the first thing I tried and it happened to work for me. From what I had read at the time, PVM should work just as well as MPI for trivially parallelizable tasks. I would not guess that the MPI module is so trivial that it did not warrant any changes, but it does look like the PVM module has seen more development activity.