When we put them before the verb in a sentence, it means “to speculate about” or “to estimate”something.
可能is more certain than 也许and we can add an adverb of degree (程度副词) before可能to speculate or estimate in a more certain way.
There are two different ways to make negative sentences, one of which is to add 不 or 没有 after 可能 and 也许.
The second type of usage in a negative sentence is to put 不 or 没有 before 可能.
Another point is that 可能can be used as a noun.