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.
I think some older instance types are still on xen, later types run kvm (code named nitro.. perhaps?). I can’t remember the exact type but last year we ran into some weird issues related to some kernel regression that only affected some instances in our fleet, turns out they where all the same type and apparently ran on xen according to aws support
