The Michelin Energy MXV4 Plus is an older tire design that's mostly used only as OEM tires today, and not very often even then. The only reason I'm guessing they keep these tires around is due to OEM contract requirements. The Energy MXV4 S8 is a newer design, and one I rather like for a passenger car, but again, it's really only an OEM tire, not often shopped in the aftermarket.
The Primacy MXV4 initially didn't have any OEM fitments, but you're starting to see some use by OEMs. Honda in particular uses this tire in at least two sizes, the 235/65R17 is used on the new Odyssey and the 235/60R18 is used on the new Pilot. Michelin has said that it intends for the Primacy MXV4 to replace the Energy MXV4 Plus and Energy MXV4 S8.
The Primacy MXM4 is, as others have noted, a newer high performance tire that is generally available only in lower profile and larger diameter sizes.