Old AWS used to be Xen, Nitro afaik uses customised VMM and I don't recall if it's not a custom OS or hosted on top of something. Azure is Hyper-V underneath I

Old AWS used to be Xen, Nitro afaik uses customised VMM and I don't recall if it's not a custom OS or hosted on top of something. Azure is Hyper-V underneath IIRC, a custom variant at least (remember Windows Server Nano? IIRC it was the closest you could get to running it), with sometimes weird things like network cards running Linux and integrating with Windows' built-in SDN facility. Rest of the bigger ones is mainly Linux with occasional Xen and such, but sometimes you can encounter non-trivial VMware deployments.