Amazon uses Xen which they call "Xen 2" I think because they've made tons of improvements and optimizations over the years.

Google uses kvm. They probably contribute as well although I haven't checked.

From a quick search Azure uses a private build of hyper-v which surprises me actually but it's possible.

Edit : millions of VMs doesn't mean anything, at the end of the day it's just servers each running a bunch of VM, which people have been doing for 20+ years. There is no scaling issue there, the hard part is storage, networking, orchestration, managed services, power...

Amazon left Xen years ago and now uses heavily customized derivative of KVM called Nitro.

Are they contributing code back to the project?

Thanks, I missed that.

What else would MS use? HyperV is very flexible. Even Xbox games are packaged up as HyperV VMs and are run in a specialised version.

Microsoft uses Azure Host OS, which indeed uses a Hyper-V variant. [0]

[0]: https://techcommunity.microsoft.com/t5/windows-os-platform-b...

For some reasons I thought they were using Linux for their hypervisor layer so Xen/KVM. Yes I think hyper-v is fine.

Google contributes KVM patches upstream. See https://www.phoronix.com/news/KVM-CPUFreq-RFC-Patches , https://www.phoronix.com/news/Google-New-ASI-KVM-2022, https://www.phoronix.com/forums/forum/hardware/processors-me...

Regarding AWS, I would also add Firecracker for AWS Lambda, which uses KVM.

I mostly agree with you. Its the "fabric" and tools that matter. In so far as this thread matters, I meant to say that VMware has a lot of great tools surrounding ESXi (which at least used to be the core of running VMs, may have changed)

In my opinion Proxmox does not have tooling like that.

So if I had written my comment better it would be what tooling and fabric these giants use and to what extent they share the tools and upgrades back to open source projects.