On top of that, it depends where. All else being equal:
-Under boxing rules, the boxer will almost certainly win, because they train for that.
-Under kickboxing rules, the kickboxer will almost certainly win, they train for that.
-Under the rules of another sport, who knows? Both of them are probably pretty much equal at Wrestling...
-Outside of a ruleset, I dunno, the guy who slugs the other one in the jaw first, while he's not paying attention? Probably depends more on who starts the fight, what range the fight starts at, whether or not both of them know there's going to be a fight...
Now, to step outside of what's considered polite, I will say this:
- Generally, I think it's easier to stay safe at longer range, if you're out matched than at a closer range. Point one for the boxer.
- Generally, I think it's easier to press and maintain a close range than it is to keep a a committed opponent at bay. In other words, I think it's easier for a boxer to keep a kickboxer at boxing range, than it is for a kickboxer to keep a boxer at kickboxing range. Point two for the boxer.
So, while it's no way a decided thing, and while I have no experience in kickboxing, and like 3 months of boxing from when I was about 15, (in other words, I have no idea what I'm talking about), I think that, all else being equal, in a sporting match with somehow mutually favourable rules, that the advantage, on average, might tend towards the boxer slightly. Like, I dunno, maybe 55-45 or something...