What is parallel virtual machine (pvm) and how it's different from mpi (message passing interface)
In fact, MPI and PVM are very different. However, they have the same objective, that is achieving data/message exchange between processors, thus enabling distributed/parallel computation.



MPI is the acronym for Message Passing Interface. It is a well-known message-passing standard, which defines the syntax and semantics of a core of library routines to exchange data between processors. Therefore, MPI is not a technology itself, but just the definitions of how to exchange data. However, MPI implementations exist in several programming languages (e.g., C, C++, FORTRAN, Python, and R), and those implementations are usually refereed simple as MPI. This fact may cause confusion since both the technology and the standards definition have the same name. 



On the other hand, PVM is a free software (released under both the BSD License and the GNU General Public License) which enables data exchange between processors, in the same way as the MPI implementations. PVM consists of a run-time environment and library for message-passing, task and resource management, and fault notification. 



You may have a look at some studies that compare both MPI and PVM. Please, see the links below. 

http://wwwmayr.informatik.tu-muenchen.de/konferenzen/Jass04/courses/2/Papers/Comparison.pdf  

https://www.researchgate.net/profile/James_Kohl2/publication/2612922_PVM_and_MPI_A_comparison_of_features/links/0c960528b9401cec40000000/PVM-and-MPI-A-comparison-of-features.pdf  

http://wwwmayr.informatik.tu-muenchen.de/konferenzen/Jass04/courses/2/Talks/Elts.ppt