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Passive and Active (Electronic) switches
KVM switches were originally passive, mechanical devices based on multi-pole switches and some of the cheapest devices on the market still use this technology. Mechanical switches usually have a rotary switch to select between computers. KVMs typically allow sharing of 2 or 4 computers, with a practical limit of about 12 machines imposed by limitations on available switch configurations. Modern hardware designs use active electronics rather than physical switch contacts with the potential to control many computers on a common system backbone.
One limitation of mechanical KVM switches is that any computer not currently selected by the KVM switch does not 'see' a keyboard or mouse connected to it. In normal operation this is not a problem, but while the machine is booting up it will attempt to detect its keyboard and mouse and either fail to boot or boot with an unwanted (e.g. mouseless) configuration. Thus mechanical KVM switches are unsuitable for controlling machines which may reboot automatically e.g. after a power failure.
Most active (electronic) KVM devices provide peripheral emulation, sending signals to the computers that are not currently selected to simulate a keyboard, mouse and monitor being connected. These are used to control machines which may reboot in unattended operation. Peripheral emulation services embedded in the hardware also provides continuous support where computers require constant communication with the peripherals.
Another problem frequently encountered with mechanical devices is a failure for all terminals to properly make contact, requiring some 'wiggling' of the switch to correct patchy colors on screen or unreliable peripheral response.
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