Per Wikipedia (so who knows):
“The USMC variant of the Osprey received the MV-22 designation and the U.S. Air Force variant received CV-22; this was reversed from normal procedure to prevent Marine Corps Ospreys from having a conflicting designation with aircraft carriers (CV).”
That's a new one. A Marine once mentioned to a few of us that it was a little bit political, with the 'M' implying it was more than just a mere 'cargo' aircraft.
Why they just couldn't call it the CV-22C I have no idea.