The gain is controlled by the duty cycle (or equivalent duty cycle, such as isolated DC-DC PWM phase shifting), such as the classic basic PWM converter Buck, Boost, Buck-Boost and its derived various isolated PWM topologies, often Using a fixed frequency to change the duty cycle control, that is, PWM modulation;
The gain is controlled by the frequency, such as the classic series/parallel/LLC resonant converter, which often uses a fixed duty cycle to change the frequency control, that is, PFM modulation;

More advanced, the gain is controlled by duty cycle and frequency at the same time, such as phase-shift LLC, resonant DAB dual active bridge, etc., which can be called PWM/PFM hybrid modulation. There are various control methods for changing the duty cycle and frequency at the same time. The trigger signal is generated based on the waveform comparison, so that the hysteresis Bang-Bang control of various topologies is treated indiscriminately, and it is only in the modulation effect.
To sum up, from a macro perspective, there are three modulation strategies for power converters: PWM, PFM, and PWM/PFM hybrid modulation. However, it should be noted that this classification is only based on the final effect of PWM waveform modulation, and does not mean that a certain topology necessarily corresponds to a certain modulation method.