P-CH mosfets are sometimes characterised showing negative voltages. So you take the gate negative with respect to the source to turn the FET on.
Normally PFETs are used with the source at the input voltage, your modulate the gate and take the output off the drain.
Another point to consider in your design, if your PWM signal is 5V, you will need to level shift the PWM signal to fully turn on and off your FET if your source is at greater than 5V.
Catches some people out...