To my knowledge AWS EC2 uses (a derivative of) Xen, Google uses KVM and Microsoft Azure uses Hyper-v. Now AWS is making inroads into KVM with Firecracker, does this mean that the days of Xen are counted?
The c5, m5, t3, c5d, m5d, z1d, r5, r5d, m5a, r5a, a1, c5n, p3dn instance families all use the Nitro Hypervisor, which is based on Linux KVM code but offloading a lot of functionality to the Nitro System instead of using the normative QEMU setup.
Firecracker runs as a process in a customer-provided Linux 4.14 or newer kernel using the upstream KVM apis, that can be on an EC2 Metal Instance or on your own hardware.
Edit: Brandan Gregg introduces Nitro (based on the KVM core kernel module) https://news.ycombinator.com/item?id=15812803 in nov ‘17