What is the difference between low energy warning and alpha floor in A320?
Low energy warning is a function of Flight Augmentation Computer (FAC). It monitors aircraft’s flight path and warns if speed is about to decay below the threshold where power is needed in order to maintain positive (ie. climbing) flight path with pitch control. 

Alpha floor is provided by auto thrust (A/THR) - (commanded by FAC), which sets full thrust (TOGA) regardless of pilot input if angle of attack rises above pre-determined angle. The threshold depends on aircraft configuration.