Nothing.
Actually, the only difference is in the translation of the Kanji to its English form. The rules of Kanji holds, that when a character (written word) ends in an n, the n is pronounced, when spoken; with the exception, that when the n is followed by another character (word), which begins with a p, the two characters are unchanged in the written form and the n is pronounced m. Ken-po follows this rule. So, if following the correct Kanji translation, it is spelled Kenpo and pronounced Kempo. It is only in transliteration that Kenpo is written Kempo.