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Virtual private server (VPS) is a expression used by Web hosting services to refer to a virtual machine. The term is used for emphasizing that the virtual machine, although operating in software on a single physical computer as other clients’ virtual machines, is functionally equal to another physical computer, is committed to the individual customer’s needs, offers the privacy of a separate physical computer, which enables it to be configured to run as a server computer (i.e. to run server software). The term virtual dedicated server or VDS is used less often for a similar concept, however it may suggest that the machine doesn’t use burst/shared memory through multiple machines, along with individual CPU cores.

In addition to decreasing hardware and power expenses, virtualisation allows companies to operate their older applications on older versions of an operating system on the same server as newer applications.

Each virtual server can operate its very own full-fledged os which enables it to be individually rebooted.

Partitioning a single server so that it appears as multiple servers has been common practice on mainframe computers and mid-range computers such as the IBM AS/400. It is now more widespread with the progression of virtualization software and technologies for microcomputers.

The physical server typically runs a hypervisor which is given the job of creating, releasing, and managing the resources of “guest” operating systems, or virtual machines. These guest os’s are allocated a share of resources of the physical server, typically in a manner in which the guest is not aware of any other physical resources save for those allocated to it through the hypervisor.

The guest system may be fully virtualized, paravirtualized, or a hybrid of the two.

In a completely virtualized environment, the guest is presented with an emulated or virtualized set of hardware and is oblivious that this hardware is not strictly physical. The hypervisor in this case must translate, map, and convert requests with the guest system into the appropriate resource requests for the host, producing significant overhead. Virtually all systems can be virtualized using this method, as it requires no changes of the os, however a CPU supporting virtualization is necessary for most hypervisors that perform full virtualization.

In a paravirtualized environment, the guest is aware of the hypervisor and connects directly with the host system’s resources, with the hypervisor implementing real-time access control and resource allocation. This translates into near-native operation since the guest sees identical hardware as the host and can thus communicate with it natively. UNIX-like systems, for instance Linux, some variants of BSD, Plan 9, and OpenSolaris are currently known to support this method of virtualization. Nevertheless, installing operating systems as paravirtualized guests tends to demand more knowledge about the os in order to have it use unique hypervisor-aware kernels and devices.

Some examples of paravirtualization-capable hypervisors are Xen, Virtuozzo, Vserver, and OpenVZ (which is the open source and development variation of Parallels Virtuozzo Containers).

Hybrid or partial paravirtualization, is full virtualization, but in which the guest uses paravirtualized drivers for key components such as networking and disk I/O, leading to significantly increased I/O performance. As a result, it is a common solution for os’s which cannot be modified (for assorted reasons) to aid paravirtualization.

One example of a hybrid hypervisor is Kernel-based Virtual Machine.

Virtual private servers fill the gap between shared hosting services and dedicated web hosting services, giving independence from other customers of the Virtual private server service in software terms but at less cost than a physical dedicated server. As a Virtual private server operates a unique copy of its operating system, clients have superuser-level access to that operating system instance, and can install virtually any software that runs on the Operating system. Certain software does not run well in a virtualized environment, for example virtualizers themselves; some VPS providers place additional restrictions, but they are generally lax compared to those in shared hosting environments. Due to the number of virtualization clients typically running on one machine, a VPS generally has limited processor time, RAM, and hard drive space.