The most important component in a Xen environment is the virtual machine monitor, also known as the hypervisor. This is the layer between the software and the hardware. The virtual machine monitor must load before any of the virtual machines start. To make that possible, you need an adjusted version of SUSE Linux Enterprise Server that immediately loads the hypervisor. This Linux version is based on a Xen-tuned Linux kernel and will start the so-called domain-0 environment. In the "Installing the Xen Domain-0" section, you will learn how to install this special version of SUSE Linux.
Since the Xen hypervisor doesn't know anything about the hardware in your machine, you need the domain-0 environment to manage hardware access. You can consider domain-0 the host operating system; it is the only instance of your operating system that has direct access to the hardware. Because of that, it is also called the privileged domain. Within domain-0, the xend process starts. This process manages all the communication with the other domains. A generic term to refer to all other domains is domain-U. These other domains are created with their own domain number, such as domain-1, domain-2, and so on.
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