Awd runs the engine and charges battery to around 25% if below that, constant awd upto 78mph then switches back to fwd, not sure if it flips back to awd when you drop back down. Normal performance with rear axle makes car feel quicker and more planted Imo. Uses rear axle in a constant manner but not full power, just gives the power to move.
Power gives the full power delivery also charges to around 25% if battery low, think of it as an enhanced sport mode, you get full power deliver to rear axle at all times upto around 78mph, full torque delivered to rear axle whenever you press the fast pedal! Makes for interesting driving, also no good for nervous passengers!
AWD is more for snow and dodgy conditions, power mode for when you want to feel your head hitting the head rest as you stab the faster pedal!
I'm not 100% sure what awd does all together, but after owning one for a while I've only used AWD in snow and on some suspect 'roads', tend to use power if the roads are quiet and want to get up to the speed limit quickly.
