These days Amazon emulates the Xen hypercall interface on top of KVM. It's pretty much their sole substantial contribution to upstream KVM (by David Woodhouse that's mentioned elsewhere in the comments in fact).

The implementation is split between KVM and their proprietary equivalent of QEMU.

We should remember to credit Oracle for starting the Xen-on-KVM work. I did start from their old patch set, even if what got merged was significantly different in the end.

I do still want to flesh out the Xen guest support in Rust-VMM at some point.

The Xen project members list suggests otherwise: https://xenproject.org/about-us/project-members/

Even if it is a custom hypervisor, AWS likely derived it from Xen and sponsors the project to continue doing so.

Since 2017, all new aws instance types have been "nitro", which is based on kvm https://www.brendangregg.com/blog/2017-11-29/aws-ec2-virtual...

AWS does continue to support older instance types, however, but only legacy workloads are using them. They actually now even run the XEN based legacy instance types on nitro. https://perspectives.mvdirona.com/2021/11/xen-on-nitro-aws-n...

It's not a custom Xen: it's *is* Xen (AFAIK, with possibly their own patch queue for some specific needs on top of it). What's custom is the toolstack around it :)

Yeah that's what I thought, but I couldn't find definitive proof besides old posts from the early days.

nitro is KVM based