check chapter 8,,,in razavi RF book about frequency synthesizers,,,also ,,there is a good book about the PLL in general that contains what you want also and very famous done by "best",,,,,,,
in brief,,PFD: when the step in frequency changes between the two compared inputs is large so that it is out of linear region ,,an constant output voltage appears which converts to ramp after passing through the loop filter and VCO "note that VCO may considered as an integrator" so, the frequency of the VCO changes greatly in the direction of the other frequency,,,till it enters the linear region( note that the linear region determined by the max. frequency step that makes the error in phase equal 2*pi),,,so,,now the output voltage of the PFD-CP will be pulsed(not constant) and its average will drive the VCO according to the loop dynamics to makes the two frequencies in phase and equal,,,,so you can consider there is two curves one for linear which detects the phase and other for non-linear which detects the frequency,,,,,,xor is just a phase detector that fails when the two inputs have significantly differences in frequencies,,,hope that help you ....
hussein