A KVM switch (with KVM being an abbreviation for Keyboard, Video or Visual Display Unit, Mouse) is a hardware device that allows a user to control multiple computers from a single keyboard, video monitor and mouse. Although multiple computers are connected to the KVM, typically a smaller number of computers can be controlled at any given time. Modern devices have also added the ability to share USB devices and speakers with multiple computers. Some KVM switches can also function in reverse - that is, a single PC can be connected to multiple monitors, keyboards, and mice. While not as common as the former, this configuration is useful when the operator wants to access a single computer from two or more (usually close) locations - for example, a public kiosk machine that also has a staff maintenance interface behind the counter, or a home office computer that doubles as a home theater PC.
A user connects a monitor, keyboard, and mouse (and audio if provided for) to the console port on the KVM device and the computers to the other ports.
KVMs can offer different methods of connecting to the computers. Depending on the port density and the type of connection, the KVM may present native connectors on the device where standard cables connect. Another method is a single DB25 or similar connector per port that connects to a special cable which has the video, keyboard and mouse connectors at the computer end. The advantage of the special cable approach is a reduction in the number of cables at the KVM. The disadvantage is the cost of these cables.
Control is switched from one computer to another by the use of a switch or buttons on the KVM device, with the KVM passing the signals between the computers and the keyboard, mouse and monitor depending on which computer is currently selected. Most electronic KVMs allow control to be switched through keyboard commands (such as hitting a certain key, often Scroll Lock, rapidly two or three times) or via an On Screen Display (OSD) menu.
Devices differ in the number of computers that can be connected, with anywhere from two up to 512 computers possible. Enterprise-grade devices can also be daisy-chained to allow even greater numbers of computers to be controlled from a single set of a keyboard, visual display unit and mouse.
A KVM switch is useful where there are multiple computers, but no need for a dedicated keyboard, monitor and mouse for each one. They are frequently used in data centers where multiple servers are placed in a single rack with a single keyboard, monitor and mouse. A KVM switch then allows data center personnel to connect to any server in the rack. A common example of home use is to enable the use of the full-size keyboard, mouse and monitor of the home PC with a portable device such as a laptop, tablet PC or PDA or a computer using a different operating system.
source:http://en.wikipedia.org/wiki/KVM_switch
