I was just rereading Water 7 now, and I came across something very intriguing that might hint towards Kizaru not having a Devil Fruit at all:
http://img39.onemanga.com/mangas/00000002/00000345/05.jpg
In the last panel on that page, Rankyaku is being shown for the first time in the manga, and notice how it's drawn - nearly exactly the same as Kizaru's attack in chapter 507:
http://img39.onemanga.com/mangas/00000002/000066782/16.jpg
So his attack might after all not have anything to do with a Devil Fruit, it could've just been an extremely powerful version of Rankyaku.
Then again, this doesn't rule out him having a Devil Fruit, because of the sound effects of his attacks being different than Rokushiki's. However I still think that even if he does have a Devil Fruit, the explosion he caused wasn't because of his Devil Fruit power.
In regards to the sound effects of the explosion and him jumping off the cannon ball, here's my take on it: Kizaru knows Rokushiki AND has a Devil Fruit. My theory is that he has some sort of Light Fruit (hence the "pika" sound effect, which implies a sparkle), and he is just using it to further enhance his Rokushiki - think of it as fusing both of his powers to create an even stronger technique.