In packet radio, AX.25 is a fairly common layer-2 protocol. Many TNCs implement AX.25 in firmware, so the operator interfaces to the TNC with a serial port, using the TNC's firmware from a shell-like interface.
Many TNCs also allow a "raw mode" where data packets are sent directly over the serial port for the host computer to manage. KISS is a common protocol for this.
XKISS is "extended KISS" and it has about 3 very simple improvements over KISS. Kantronics TNC documentation indicates that it was conceived by John Wiseman, G8BPQ. Kantronics TNCs provide an XKISS mode, in addition to a standard KISS mode.