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...
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...