Pixels – corresponds to actual pixels on the screen.
Density-independent Pixels – an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion.
Scale-independent Pixels – this is like the dp unit, but it is also scaled by the user’s font size preference. It is recommend you use this unit when specifying font sizes, so they will be adjusted for both the screen density and user’s preference.
In my experience, never use px in android xml layout file (except special case). You should use dp unit instead. Since sp unit is preferred to the font size which was set on real device by user, you should use this measurement when defining text size. But, never use it for layout size.