Edit: also "interrupt-based" in this case has nothing to do with interrupts seen by the CPU. Difference is that in ps/2 the device can send data to the controller (which then generates interrupt) at any time, while for usb the controller periodically polls devices for data (and possibly generates interrupt if there are some). In the early days of USB and UHCI host controllers this polling was done in software, but since USB 2.0 this is done in hardware and generates real cpu interrupts when usb device requests interrupt (although with somewhat unpredictable but bounded latency)