There are a few main virtualization types: KVM, VMWare, OpenVZ and Xen. Out of them, we chose KVM because it has been proven to be the fastest. This is mostly due to how highly integrated it is with the hypervisor’s kernel.
Nested KVM is not supported at this time. It is still a young concept and we will continue to watch for it to be production ready. You can however purchase Citrix XenServer and run Xen within our cloud.