What is the difference between pitch detection and Onset detection?
The two concepts are related to two different dimensions or aspects of music which might or might not be correlated.

Onset detection is concerned with finding the points in time at which sounds start. Doing this does not require prior knowledge of the particular pitch (or fundamental frequency) of the sound. It may indeed rely on the property that at the beginning of a sound, there is an increase of energy. Actually, you can very well perform onset detection on recordings which do not have a well-defined pitch (for example: drums, machine noises...).

You can think of pitch and onsets/rhythm as related to the "vertical" and "horizontal" organization of music. There exists types of material in which changes along these two dimensions coincide - so that a detection algorithm can take advantage of a change in one dimension to infer that a change has occurred on the other dimension. But this coincidence/correlation is contingent.