The Yavin Space Station is in both versions of the game (XBox and PC). BUT on the XBox version of the game, it is an upgrade that you had to download to your XBox using XBox Live. I just don't know if that is still possible.
I believe that on the PC version of the game it is accessible right from the beginning (as soon as you have access to the Ebon Hawk), but I'm not sure, so I would like for someone with the PC version to confirm that.
F.Y.I., Yavin is an extra "world" (a big shop, really) that contains tons of exotic/powerful/cool items that aren't available anywhere else. From what I hear, they are expensive but very powerful, and you get access to more items as you progress through the game. It's a shame that they didn't bother to put it on the XBox disc.
