AWS' Nitro platform is no longer based on Xen and the Xen stuff is being replaced/deprecated, including moving some of the old Xen instance types to run on Nitro instead of Xen.
If you mean Amazon, it's not really been based on Xen (although it's still used in places) for a long time. They largely use their own hypervisor now.
Do you know, is it an offshoot of xen or kvm, if at all?