Just to make it clear, do You actually want to understand how circuitry in servo works, or how to control it using PWM?
In both cases Google is Your best friend. Servo control by PWM is explained in my previously posted links; as for servo internals, Google
helps a lot; one of the first Google suggestions is How RC Servos Works
article. I'm not sure, but I think that HS-311 operates the same way any other standard analog servo does: pot is used to determine current angle, normal DC motor with gearbox to output rotational movement, some IC to: detect PWM and translate to angle, check pot voltage and compare it against PWM value, output voltage to control motor.