The protocols used in building PVM are evolving, with the result that newer releases are not compatible with older ones. Compatibility is determined by the pvmd-task and task-task protocol revision numbers. These are compared when two PVM entities connect; they will refuse to interoperate if the numbers don't match. The protocol numbers are defined in src/ddpro.h and src/tdpro.h (DDPROTOCOL, TDPROTOCOL).
As a general rule, PVM releases with the same second digit in their version numbers (for example 3.2.0 and 3.2.6) will interoperate. Changes that result in incompatibility are held until a major version change (for example, from 3.2 to 3.3).