AWD as applied to different makes can mean different things. For example, in the Audi Quattros in use by my family, the Quattro AWD is active all the time with nearly 50%/50% power distribution F/R and is aimed at overall performance handling in dry as well as wet/snow conditions. In the Nox/Terrain application, the AWD is always there but its intent is solely for traction improvement and works in conjunction with traction control. In the Nox/Terrain, power is applied nearly "100%" to the front unless wheel slippage is detected whereby traction control is engaged and power is also transferred automatically to the rear via computer control using an algorithm particular to the vehicle. In the Nox/Terrain it is NOT user-controlled and is designed to be seamless to the driver; the driver throttles up and if it's slippery the rears get more and more power to get the vehicle going. The system is designed for on-road traction in the case of the Nox/Terrain. The fuel economy reduction is due to the extra weight combined with parasitic losses added by the AWD drivetrain (the driveshaft to the rear and the differential in the rear).
As an aside, the V6 gives much less fuel economy than the I4 on the highway more due to the parasitic frictional losses from the extra 2 cylinders and different, heavier-duty transmission than to the extra weight or arguable engine design issue. The difference is less dramatic in the city cycle. At 60mph, a V6 Nox/Terrain needs the same horsepower to maintain speed as does an I4, all else being equal. So, at 60mph (flat level ground) the fuel economy hit from having the V6 over the I4 is due mostly to the extra friction from the 50% increase in moving engine internals (4cyl + 50% = 6cy) and the heavier-duty transmission internals. Adding AWD to both also adds parasitic frictional forces that reduce economy.
I have been waiting for some unclassified schematics to come out of GM's Tech Center regarding the AWD system and when they are OK for release I'll post what they give me; I have some friends in engineering on the inside there and they are very good about confidentiality so I have to respect that. Hope the info I gave helps a bit.