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Xen Hypervisor

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PCQ Bureau
New Update

As the name suggests this open source GNU licensed virtual machine monitor or hypervisor sits directly on underlying hardware. And like other hypervisors this one too lets you consolidate multiple servers into one thus saving cost by optimally utilizing hardware. One can run different types of operating systems including Windows (there is a catch however, which we shall discuss later) on virtual machines created on top of this hypervisor. If one has to compare this hypervisor with the well known VMware ESX/ESXi server, we shall find lots of differences.

Let me throw some light on the actual components of the Xen environment. There are three important components: the first is a hypervisor that sits on hardware, which is responsible for CPU scheduling and memory partitioning of various virtual machines running on the hardware device. The hypervisor controls the execution of virtual machines as they share common processing environment. The second component is Domain 0, which is a modified Linux kernel, a unique virtual machine running on the Xen hypervisor that has special rights to access physical I/O resources as well as interact with other virtual machines (Domain U: PV and HVM guests) running on the system. All Xen virtualization environments require Domain 0 to be running before any other virtual machine can be started. This means a single point of failure which is a negative point for Xen. Two drivers are included in Domain 0 to support network and local disk requests from Domain U PV and HVM Guests: the Network Backend Driver and the Block Backend Driver. The final component Domain U guests do not have direct access to physical hardware on the machine (as Domain 0). All paravirtualized virtual machines running on a Xen hypervisor are referred to as Domain U PV Guests and are modified Linux operating systems: Solaris, FreeBSD, and other UNIX operating systems. All fully virtualized machines running on a Xen hypervisor are referred to as Domain U HVM Guests and run standard Windows or any other unchanged operating system. The Domain U PV Guest virtual machine is aware that it does not have direct access to the hardware and recognizes that other virtual machines are running on the same machine. The Domain U HVM Guest virtual machine is not aware that it is sharing processing time on the hardware and that other virtual machines are present. A Domain U PV Guest contains two drivers for network and disk access, PV Network Driver and PV Block Driver. A Domain U HVM Guest does not have the PV drivers located within the virtual machine, instead a special daemon is started for each HVM Guest in Domain 0, Qemu-dm. Qemu-dm supports the Domain U HVM Guest for networking and disk access requests.

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