AWS HVM vs PV

HVM: Hardware Virtual Machine
PV: Paravirtual machine

HVM is newer (feature of new CPUs). Some of the newer instance types only support HVM.

References:

• http://palakonda.org/2012/10/30/aws-virtualization-hvm-vs-paravirtualization/
• http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/virtualization_types.html