Resposta curta: escolha o HVM, porque a Amazon sugere isso.
Essas duas imagens são para diferentes tipos de instâncias. A partir dos Tipos de Virtualização da AMI do Linux , você entenderá a diferença entre esses dois tipos.
HVM:
This virtualization type provides the ability to run an operating system directly on top of a virtual machine without any modification, as if it were run on the bare-metal hardware.
PV:
Paravirtual guests can run on host hardware that does not have explicit support for virtualization, but they cannot take advantage of special hardware extensions such as enhanced networking or GPU processing.
No documento do AWS EC2, a Amazon alega que não há diferença significativa de desempenho entre esses dois:
Historically, PV guests had better performance than HVM guests in many cases, but because of enhancements in HVM virtualization and the availability of PV drivers for HVM AMIs, this is no longer true.
Portanto, escolha HVM.