A KVM (with KVM being an abbreviation for Keyboard, Video, 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.
A user connects a monitor, keyboard, and mouse to the KVM device, then uses special cables (generally USB and VGA ) to connect the KVM device to the computers. 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 devices also allow control to be switched through keyboard commands (such as hitting a certain key, often Scroll Lock, rapidly two or three times)
Devices differ in the number of computers that can be connected, with anywhere from two up to 64 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, video and mouse.