Maybe because xen is a type 1 hypervisor in its original meaning and all the other ones are type 2? (yes, ESX(i) doesn't use linux but it also brings its own os

Maybe because xen is a type 1 hypervisor in its original meaning and all the other ones are type 2? (yes, ESX(i) doesn't use linux but it also brings its own os on which it runs on top)