Amplitude modulation modifies and changes the amplitude of the carrier so that it represents ones or zeros. A one represents that a carrier is present whereas a zero shows that a carrier is not present. The main advantage of amplitude modulation is that it is very simply to design but it also comes with its disadvantages which are that any loss of connection will always be read as a zero. The other disadvantage of amplitude modulation is any noise spikes that are given off on a transmission medium can interfere with carrier signals.
Frequency modulation modifies the amplitude of the carrier so that the signals represent the ones or zeros, pretty much the same as amplitude modulation. Frequency modulation comes with two advantages which are an immunity to noise on the transmission medium and that there will always be a signal present, in other words, any loss of connection will be detected early on. There are also two disadvantages which are that two frequencies are required and that a detection circuit will be needed to recognize both frequencies in the event that the signal is lost.
Phase modulation also modifies the carrier so that it represents ones and zeros. The carrier phase is changed every time a one occurs but will not change whenever there is a zero. There are two advantages to this type of modulation which are only one frequency is used, making it better than frequency modulation, and that it is very easy to detect a loss of carrier. The only disadvantage of phase modulation is that it is made up of complex circuitry which is required to detect and generate changes in phase.