PSK is a linear modulation technique as it is generated by a linear methods, eg. the phase of the carrier is changed linearly with the input modulating signal, same is true for the ASK where amplitude is varied linearly according to the input information bearing signal. The same applies to QAM modulation techniques which are combination of both PSK and ASK. On the otherhand FSK type modulation techniques are non-linear due to the fact these are generated non-linearly by having the phase of the carrier generated by integrating the input signal (to be modulated).