Xen provides fast, secure, open source virtualization that allows multiple operating system instances to run as Xen Virtual Machines or VMs on a single physical x86 computer. Xen supports modified guest operating systems using a technique known as paravirtualization, which requires modifying the operating system to run on Xen, but offers near-native performance. Paravirtualized operating systems "know" that they are virtualized. Xen also supports unmodified operating systems using processor extensions from Intel (VT) and AMD (AMD-V).

Xen supports 32-bit processors with and without Physical Address Extension (PAE), 64-bit processors, and Symmetric Multiprocessing (SMP) guest operating systems.

Xen is exceptionally lean, which leads to extremely low overhead and near-native performance for VMs. Xen re-uses existing Linux device drivers for Linux VMs, and uses special paravirtualized device drivers for network and disk I/O on Windows VMs, making device management easy. Moreover, Xen is robust in the event of device driver failure and protects VMs, and also protects the hypervisor from faulty or malicious drivers.

Xen provides superb resource partitioning for CPU, memory, and block and network I/O. This resource protection model leads to improved security because VMs and drivers are not susceptible to denial of service attacks. Xen is fully open to scrutiny by the security community and its security is continuously tested. Xen is also the foundation for a Multi-Level Secure System architecture being developed by XenSource, IBM and Intel.

Xen was originally developed by the Systems Research Group at the University of Cambridge Computer Laboratory as part of the XenoServers project, funded by the Engineering and Physical Sciences Research Council (EPSRC), the main funding agency in the United Kingdom for research in engineering and the physical sciences as well as the managing agent on behalf of the other Research Councils for High Performance Computing.

XenServer allows IT administrators to flexibly assign up to 16 x86 servers into a single Resource Pool of server resources. Multiple Pools can be managed from a single XenCenter management console. A Resource Pool is a tightly coupled collection of servers whose resources are virtualized to host a set of Virtual Machines. Servers in a Resource Pool monitor the configuration state and availability of their peers. XenServer management state is also replicated across all servers in a Pool, with the benefit that failure of a Pool master can be quickly remedied, since any node in the cluster can replace the failed node. Using the XenServer clustering architecture, the workload of a cluster can be protected from server failures, through a unique combination of shared storage, Xen virtualization, and replicated state management between servers in the cluster.

Virtual Machines assigned to a Resource Pool are automatically mapped onto the physical resources of the Pool, but IT administrators retain full control of resource assignment, and full visibility into each system and each Virtual Machine, including the ability to manually place workload on specific servers, and drill down into each server within the Pool to get a precise view of each server's resources and the Virtual Machines it hosts. At the simplest level, all the administrator needs to do is assign a Virtual Machine or a set of Virtual Machines to a Resource Pool. XenServer manages the rest, including the assignment of physical resources from servers in the Pool to host the VMs, and ensuring that administrator policies for resilient restart of VMs are implemented. XenServer ensures that the overall utilization of the resources of the servers in the Pool is maximized, to deliver lowest possible TCO. Of course, if you want to assume full control, XenServer gives you the ability to manage each resource for each VM, but most users will appreciate the simplicity of the "drag and drop" interface for VM provisioning with guaranteed VM performance, automated VM storage and network management, and the use of policies for automatic restart on failure of physical components of the cluster.

In most datacenters, storage is managed as a shared, separately administered resource independent of the different server applications and OS types that make use of it. The rich set of choices for datacenter storage, and the emergence in its own right of storage virtualization as a powerful technology that reduces TCO for storage, leaves IT managers with a bewildering set of choices for storage and storage management. XenServer aims to simplify the management of diverse storage technologies for virtualized infrastructure. It does this by

XenServer, uniquely amongst all virtualization products on the market, offers an open API to integrate directly with the various kinds of storage infrastructures available. With built-in support for IDE, SATA, SCSI and SAS drives, XenServer can manage all forms of storage local to any server in a Resource Pool. Through NAS, iSCSI and SAN support, XenServer extends the available Virtual Machine storage options to the most common datacenter architectures in use today, while providing plug-in APIs for storage vendors to integrate any storage management technology, from clustered file systems, through clustered volume management. Storage Repositories (or SRs) are elemental components of the XenServer architecture. All are managed via the XenServer API, and through this API XenServer can leverage the built-in features of the underlying storage infrastructure, including snapshotting, backup, and automated creation and assignment of LUNs for new Virtual Machines. Not all SRs support all primitives - but XenServer can accommodate this by adding software-level features that can be used if the storage infrastructure cannot support a particular primitive, such as snapshotting.

When a XenServer Host in the Pool needs physical maintenance, VMs can be relocated to other servers in the Pool, while they are running, with only hundreds of milliseconds of observable delay. This live relocation capability is called XenMotion.

The three variants available are

Each member of the XenServer product family provides the XenCenter management interface and a full set of product documentation.

XenServer contains all you need to quickly and easily set up a virtualized Xen environment: