The cap is lvl 30 ?
Regardless, the game is open-ended and doesn't end after the campaign so there's technically unlimited xp available
The devs have said that there is a soft level cap, meaning that XP returns will decrease as you level. Eventually you will reach a point where you will simply not get XP from most enemies, and I suspect that the ones that will give you decent XP at high levels are unique (e.g. dragons).
Either respawning enemies won't give any XP once you reach a certain level, or they will give so little XP that it will take ages (months, perhaps, depending on the respawn rate) to reach the next level. If the latter is true, there would technically be unlimited XP, but it would be practically worthless. However, the first option would mean there is a limit to XP, at least until such time as they release DLC. You'd still be able to roam around and kill things, but you wouldn't receive XP for doing so.
ETA: totally ninjaed because I'm way too verbose 