Get the patches from netlib. By convention, they go in $PVM_ROOT/patches and are named, e.g., pvm334to5 (converts version 3.3.4 to 3.3.5). To install them, you'll need to have the patch command (free software) installed on your system.
To install a patch, change to the $PVM_ROOT directory and type e.g.: patch -p0 < patches/pvm334to5. Check the file patches/README for any special instructions.