The shadow pvmd (pvmd') runs on the master host and is used by the master to start new slave pvmds. Any of several steps in the startup process (for example, starting a shell on the remote machine) can block for seconds or minutes (or hang), and the master pvmd must be able to respond to other messages during this time. It's messy to save all the state involved, so a completely separate process is used.
The pvmd' has host number 0 and communicates with the master through the normal pvmd-pvmd interface, though it never talks to tasks or other pvmds. The normal host failure detection mechanism is used to recover in the event the pvmd' fails. The startup operation has a wait context in the master pvmd. If the pvmd' breaks, the master catches a SIGCHLD from it and calls hostfailentry(), which cleans up.