Used to be that only the Windows instances were HVM (thus the defenestration hack using them to get FreeBSD onto AWS) and all the Unix ones used paravirtualization - has this changed?
Cluster, high-I/O, and M3 all have HVM available. Basically, all the new instance types released since I started complaining to EC2 engineers about how HVM is so much easier to work with than PV... ;-)