Let's see, where to begin? Well, first off, I'm not that much up to date with Japanese history at all, so I wouldn't know about Kessen 1. Now, about Kessen 2 and the Dynasty Warriors series; actually, they are both based on the novel (which is based on history... but my point is that both follow the novel more than they do history), to an extent, and when I say to an extent, I mean that some of the personalities that you will find in the novel (Zhang Fei's drinking habits, Liu Bei's noble factor, Zhuge Liang's godlike abilities, Cao Cao's seeming ambition to rule all of China, etc.) are present in Kessen 2, while many, many, many things are completely made up or altered to follow the game's storyline (such as the female generals under Cao Cao (who were originally males in history), Cai Wengi as a ruler, heck, Himiko, who in the end flees to Japan (there was an actual queen/princess/empress? by the name of Himiko in Japan during that time period who traded with some of the Chinese, if I recall correctly)). The Dynasty Warriors series follows the novel more closely than it does history as well. Such as all the who killed who instances, the duels (which hardly ever can be found outside of the novel), heck, most of the cutscenes follow the novel exclusively.
Whoo, that was long... sorry, just had to clear that up...