You can't measure voltage between pulses with a PIC... not only does the acquisition time prohibit this, it's unnecessary.
The average PWM voltage will regulate the speed as long as the load is fairly constant. If this is an RC plane, prop load is fairly constant. In some cases, current regulation is also useful, but voltage is what you're looking for.
So, what you need here:
1. Take the voltage on the motor and feed it through a voltage divider, about 3:1 (output can't be over 5V). Output impedance needs to be a max of 2.5k. Add a filter cap of the proper value for the PWM frequency so that you have an average DC value of the output.
2. PWM output needs to go to a driver, a low side driver won't work with the divider, so you'll need an NMOS, a PMOS, and a resistor.
A simpler solution would be to sense the power supply voltage with the divider, and base your PWM solution on that. In this case, you can use only a single NMOS for the PWM driver. This provides somewhat poorer regulation than output sensing, but if this is an RC plane the weight is the critical factor.
Now keep in mind that the PWM can only lower the average output voltage. It can't boost voltage. That is, if you want it to run as fast at 11V as it does as 13V, the solution is to leave it 100% on at 11V and pulse it about 85% "on" when you've got 13V so it appears as 10V to the motor.