What important terminology should I know?

What is a hypervisor?
The hypervisor is the most basic virtualization component. It's the software that decouples the operating system and applications from their physical resources. A hypervisor has its own kernel and it's installed directly on the hardware, or "bare metal."  It is, almost literally, inserted between the hardware and the OS.

What is a virtual machine?
A virtual machine (VM) is a self-contained operating environment—software that works with, but is independent of, a host operating system. In other words, it's a platform-independent software implementation of a CPU that runs compiled code. A Java virtual machine, for example, will run any Java-based program (more or less). The VMs must be written specifically for the OSes on which they run. Virtualization technologies are sometimes called dynamic virtual machine software.

What is paravirtualization?
Paravirtualization is a type of virtualization in which the entire OS runs on top of the hypervisor and communicates with it directly, typically resulting in better performance. The kernels of both the OS and the hypervisor must be modified, however, to accommodate this close interaction. A paravirtualized Linux operating system, for example, is specifically optimized to run in a virtual environment. Full virtualization, in contrast, presents an abstract layer that intercepts all calls to physical resources.

Paravirtualization relies on a virtualized subset of the x86 architecture. Recent chip enhancement developments by both Intel and AMD are helping to support virtualization schemes that do not require modified operating systems. Intel's "Vanderpool" chip-level virtualization technology was one of the first of these innovations. AMD's "Pacifica" extension provides additional virtualization support. Both are designed to allow simpler virtualization code, and the potential for better performance of fully virtualized environments.

What is application virtualization?
Virtualization in the application layer isolates software programs from the hardware and the OS, essentially encapsulating them as independent, moveable objects that can be relocated without disturbing other systems. Application virtualization technologies minimize app-related alterations to the OS, and mitigate compatibility challenges with other programs.

What is a virtual appliance?
A virtual appliance (VA) is not, as its name suggests, a piece of hardware. It is, rather, a prebuilt, preconfigured application bundled with an operating system inside a virtual machine. The VA is a software distribution vehicle, touted by VMware and others, as a better way of installing and configuring software. The VA targets the virtualization layer, so it needs a destination with a hypervisor. VMware and others are touting the VA as a better way to package software demonstrations, proof-of-concept projects and evaluations.

What is Xen?
The Xen Project has developed and continues to evolve a free, open-source hypervisor for x86. Available since 2003 under the GNU General Public License, Xen runs on a host operating system, and so is considered paravirtualization technology. The project originated as a research project at the University of Cambridge led by Ian Pratt, who later left the school to found XenSource, the first company to implement a commercial version of the Xen hypervisor. A number of large enterprise companies now support Xen, including Microsoft, Novell and IBM. XenSource (not surprisingly) and SAP-based startup Virtual Iron offer Xen-based virtualization solutions.

What are the cost benefits of virtualization?

IT departments everywhere are being asked to do more with less, and the name of the game today is resource utilization. Virtualization technologies offer a direct and readily quantifiable means of achieving that mandate by collecting disparate computing resources into shareable pools.

For example, analysts estimate that the average enterprise utilizes somewhere between 5 percent and 25 percent of its server capacity. In those companies, most of the power consumed by their hardware is just heating the room in idle cycles. Employing virtualization technology to consolidate underutilized x86 servers in the data center yields both an immediate, one-time cost saving and potentially significant ongoing savings.

The most obvious immediate impact here comes from a reduction in the number of servers in the data center. Fewer machines means less daily power consumption, both from the servers themselves and the cooling systems that companies must operate and maintain to keep them from overheating.

Turning a swarm of servers into a seamless computing pool can also lessen the scope of future hardware expenditures, while putting the economies of things like utility pricing models and pay-per-use plans on the table. Moreover, a server virtualization strategy can open up valuable rack space, giving a company room to grow.

From a human resources standpoint, a sleeker server farm makes it possible to improve the deployment of administrators.