PWM isn't ordinarily thought of as improving a motor's efficiency. In fact, since the motor would, essentially, lose speed and therefore torque during non-powered pulses, the energy needed to overcome lost inertia and return the motor to full speed/torque would
decrease efficiency.
Rather, it's a means for greater and more precise control of the motor's speed or, in the case of a servo, it's position.
For a resistive load (a heater element, for instance) PWM would control the level of heat produced but, similar to the above case, would not increase efficiency.
Efficiency is increased/decreased by removing/adding parasitic loads.
So, with PWM control, you've
also added the parasitic load(s) of the PWM control components.
<EDIT> Stupid typos from utter lack of touch typing ability...