Permanently hide Navigation Bar in an activity

You can

There are Two ways (both requiring device root) :

1- First way, open the device in adb window command, and then run the following:

 adb shell >
 pm disable-user --user 0 com.android.systemui >

and to get it back just do the same but change disable to enable.

2- second way, add the following line to the end of your device's build.prop file :

qemu.hw.mainkeys = 1

then to get it back just remove it.

and if you don't know how to edit build.prop file:

• download EsExplorer on your device and search for build.prop then change it's permissions to read and write, finally add the line.
• download a specialized build.prop editor app like build.propEditor.
• or refer to that link.

There is a solution starting with KitKat (4.4.2), called Immersive Mode: https://developer.android.com/training/system-ui/immersive.html

Basically, you should add this code to your onResume() method:

View decorView = getWindow().getDecorView();
decorView.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                              | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                              | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                              | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                              | View.SYSTEM_UI_FLAG_FULLSCREEN
                              | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY);