The 1996 ARRL Handbook, Chapter 22, page: 22-47 has Kenwood Interface schematics to DIY build a Kenwood radio to computer RS-232 serial interface. 6-pin DIN to RS-232 (DE-9 connector).
The interface uses the Maxim MAX232 IC (serial signal level conversion) and PS-2501-1NEC  (signal inversion and isolation) for TxD, RxD, CTS, and RTS lines from the 6-pin DIN on the Kenwood radio.
Only 5 pins are used on the 6-pin DIN, the 4 previously mentioned data and control lines, plus Signal GND.
Universal Serial Bus (USB) conversion from the radio's 6-pin DIN is a bit more complex.
You need to use a serial UART to USB, such as the FTDI FT230X USB Bridge IC http://www.ftdichip.com/Products/ICs/FT230X.html
and then use a hex inverter (7404) or isolation / inversion components (as above) required for these Kenwood radios.http://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT230X.pdf
FTDI is one of the better mfg. for USB Bridge ICs and the industry's best VCP (Virtual COM port) drivers for various computer operating systems (OS).