A keyboard-mouse-video (KVM) switch has a server interface, a client interface, a switch circuit and a digital video overlapping circuit. The server interface is connected to plural computers, and the client interface is connected to plural sets of manipulation and display devices. The switch circuit routes paths between the computers and the sets of manipulation and display device. The digital video overlapping circuit overlaps a digital video overlapping image onto a digital video signal received from one of the computers through the server interface. The overlapped digital video signal is then transmitted to one of the sets of manipulation and display device through the client interface.