Virtuozzo is a software application for enterprise server virtualization that allows an administrator to create virtual environments on a host computer at the operating system (OS) layer.
Instead of having one physical machine run multiple operating systems simultaneously, as the virtual machine model used by VMware and Microsoft Virtual Server or XEN does, Virtuozzo approaches virtualization by running a single OS kernel as its core and exporting that core functionality to various partitions on the host. Each of the partitions effectively becomes a stand-alone entity called a virtual private server (VPS).
Virtuozzo comes with a proprietary Kernel Service Abstraction Layer (KSAL) that manages access to the kernel and prevents any single VPS from bringing the entire physical server down. It also has a proprietary file system to completely isolate the partitions, a security precaution that prevents a software fault in one partition from impacting an application or data in another partition.
Every VPS has its own network address and its own set of login credentials, system processes and daemon services. Because the underlying operating system is always running, each VPS can be rebooted independently and data can be migrated from one virtual environment to another on a live host. This capacity -- which is not possible with the virtual machine model -- is one reason why Virtuozzo has the reputation for being a good choice for production servers working with live data and applications.
Virtuozzo's architecture eliminates system calls between layers, which reduces CPU usage overhead dramatically. Common binaries and libraries on the same host machine can be shared, making it possible for administrators to put literally thousands of VPSs on the same machine, each functioning as a stand-alone server.
Administrators using Virtuozzo have three management options: the command line, Virtual Management Center (a GUI interface for management and monitoring) and Virtual Control Center (a Web-based interface for remote access).
Virtuozzo is created and distributed by SWSoft. In 2006, SWsoft released the core of Virtuozzo under the GNU GPL (General Public License) in an open-source project called OpenVZ.