What is the advantage of Xen Virtualization

VMware and KVM provide a kernel module for Linux that allows the HVM to take place. The First Operating System to boot is usually referred to as the host O/S. Both KVM and VMware have their own way of doing this. In VMware, it has a proprietary set of libraries that connect things like storage and input/output or shared network devices. KVM uses Qemu to provide these things instead of having an internal set of libraries.

 

Xen on the other hand does things a little bit differently. Xen is the host which first gets loaded and installs itself as a thin layer between all of the operating systems and the hardware. The first O/S it loads after its own initialization is given the host permissions and then considers the host operating system. That O/S is referred to as the Domain-0 or "Dom-0". Xen takes all the hardware in the machine and defines what is accessible to each virtual machine based on what instructions it is given by the Dom-0 VM. 

 

In terms of performance, Xen, VMware ESX, VMware Hypervisor and Vsphere are all the highest performing as they represent the "Hypervisor" type of Virtualization. VMware Player, VMware Server and KVM are the close second in performance as they still use Hardware Virtualization, but have the "host" O/S under them, taking a minor amount of performance from the guests. Last are the emulators, such as unmodified VirtualBox and QEMU doesn't use KVM. These are the lowest performing methods as they must translate all CPU instructions before executing them, whereas the Hardware VM types can simply pass them down to the CPU directly.

 

On the other side of the coin, emulators like Qemu and VirtualBox are not restricted to running an O/S that is built specifically for the hardware you're running. For example, KVM cannot run a PowerPC operating system (like the old Mac OSX) on an Intel based computer. It can run newer versions of OSX that are Intel Compatible.