Incredibly there's a line between those two places!
The bus is called the Fujiyama Liner. You catch it at the Hachijo-guchi Exit of the Kyoto Station.
The ride will be about 9 hours and it costs 8,500 yen (one way).
Q-NAVI
EDIT: The fare seems to be 8,000 yen instead of 8,500. The bus leaves Kyoto at 23:18 and arrives at Kawaguchi-ko at 8:20.