I believe it's because Xen adds overhead[0] in its process of working around the need for hardware virtualisation support[1], whereas KVM has much less overhead

I believe it's because Xen adds overhead[0] in its process of working around the need for hardware virtualisation support[1], whereas KVM has much less overhead, but requires hardware support to run efficiently. [0]: http://dtrace.org/blogs/brendan/2013/01/11/virtualization-performance-zones-kvm-xen/ http://dtrace.org/blogs/brendan/2013/01/11/virtualization-pe... [1]: Xen was built before such support was widely adopted.