Aliyun and Tencent use Xen, too. They are both on Xen's pre-disclosure list. I don't know it now, but Aliyun was at least Xen-only at one point. Huawei offers products both with KVM and Xen. This doesn't necessarily invalidate your speculation though.

The OpenStack aspect is true. Xen lacks support there.

A new Xen guest mode called PVH will remove QEMU when running Linux -- it is basically HVM without QEMU. Windows still requires QEMU.