This is plain Xen + their own home-made "secret" toolstack (built year after year).
They also implement features on their side, sometimes valuable stuff, sadly it seems there is a lack of sharing. And because Xen is GPL and not aGPL, they are not forced to give it back.
Sometimes, you see Xen people developing a feature which is already working in Amazon. That's a bit sad when you have to do things twice, but you know, big corps...