Android 4.4 KitKat comes with Immersive mode - true full-screen mode for apps
Such is the case with the new Immersive mode, which will allow developers to take advantage of the full screen space with their apps and games. We know what you're thinking - wasn't this the case already? And the answer is no, because most of the time there was either a status bar on top, or Android's navigation buttons taking up precious space. That was quite annoying for owners of phones like the Xperia Z1, for example, which has a massive, 5" display, but a significant part of it has always been occupied by three on-screen buttons.
source: Google
Things that are NOT allowed: