Difference between a constant DC and PWM
I was wondering what would you get on subtracting a 12V (From 0 to
  12V) PWM (50% duty cycle), from a 12V constant DC source?


Well half the time you'd get 0V (12V - 12V = 0V) and the rest of the time you'd get 12V (12V - 0V = 12V). 

In fact what the subtraction does is invert the squarewave about a mean level of 6V but, because it has a 50% duty cycle it looks exactly the same.

If instead you had a 30% duty cycle (12V 30% of the time) you'd end up with a waveform that has 12V 70% of the time.