I'm not quite sure the status of it at least, but reported back in 2017 that they are moving off Xen

https://www.theregister.com/2017/11/07/aws_writes_new_kvm_ba...

It could be that it's not all over and tied to specific machine types still, or there's something they've done to make it report to the guest still that it's xen based for some compatibility reasons.