With PWM the longer the "on" time the higher the effective voltage, resulting in increased RPMs, this part of a brushless ESC is essentially the same as in a brushed controller, though brushed controllers tend to use a lower frequency for PWM.
The PWM is performed before the commutation/switching part of a brushless ESC, the PWM is the means used to control the speed by varying the effective voltage provided to the motor. With PWM the width of the pulse increases as the throttle is increased until the the DC voltage is "on" all the time at full throttle, this is the opposite of an AC Thyristor type control/dimmer that cuts off the voltage at variable levels to reduce the average voltage.
Brushless motors are commutation mode motors just as any brushed motor, only the mechanical commutation has been replaced by electronic commutation provided by the controller.
True AC motors do not have a Kv rating, as they are controlled by the frequency of the AC current, not the voltage applied. No AC motor on line or mains current can exceed 3600 RPM (60Hz) and induction type motors "slip" about 50 RPM below their synchronous speed at no load. Thats why brushed series wound motors are used for so many applications on AC, it's the only way to get 30K RPM out of your Dremel tool or any other appliance that requiers higher RPMs than AC motors can provide. Series wound brushed motors while used mainly on AC today, are classed as "universal" motors as they will run on AC or DC, brushed permanent magnet motors on the other hand run only on DC.
Regards,
Pete