At first the notion that they might not be didn't even occur to me. But then I got to thinking about who's producing this game.
Matt Firor was the lead on Dark Age of Camelot. For those not in the know, DAoC also had 3 "provinces" (Realms). Midgard, Albion and Hibernia. A player from Midgard could not enter Hibernia or Albion, and so on. Each of the 3 realms met to do battle in their three respective Frontiers. This is where the Realm vs. Realm/PvP took place.
ESO has 3 realms with 3 factions. It's being made by the same guy who did DAoC. Have they actually said anything specifically about travel between the provinces? I know the notion of not being able to sounds bad, but RvR in DAoC was amazing, and the realms were huge enough to keep you busy for a long time.
This is honestly something that hasn't come up or been mentioned by Matt Firor or anyone else at ZeniMax Online. It's a possibility ESO may be going a similar direction in regards to PvP that DAoC did, but at the same time I'm hesitant as this is a TES game. One of the core pillars of TES is to
go wherever you want and limiting exploration based on faction would put a damper in that well-established core element of TES.
I never played DAoC personally, but from my understanding that was mainly a PvP-only MMO. Since ZeniMax Online is also focusing heavily on PvE, that may be something they had considered while in development and could have potentially modified the DAoC model. It's honestly just something we don't know and we'll need to hear confirmation from the developers. Hopefully the GI podcast will ask this question in regards to exploring Tamriel.