As others have explained, "可能" and "有可能" are used interchangeably in Mandarin. Messquito suggests otherwise, but I think in the examples he gives, whether "可能" means "probably" or "possibly" totally depends on the speaker's tone. Then again, I'm from Beijing, maybe they are different in Mandarin spoken in Taiwan and other places.
"可能" originally meant "may be able" (可=may, 能=able).
The phrase then became a word that can be used as an adjective (possible, probable), an adverb (possibly, probably) or an auxiliary (can, may) depending on context.
Its use as a noun to mean possibility, as in "有可能 (there is possibility)", occurred in modern Chinese. A large amount of foreign literature came into China in the early 20th century following the fall of imperial China. Because the grammar of 汉语 is quite different from most other languages, the practice of using adj., adv. and aux. as nouns, or changing them to nouns (by adding the suffix "性", e.g. 可能性, 可行性, 复杂性, 随意性), became common when translating foreign literature. This is only one of the many influences foreign languages have had on the development of 汉语.
So, "可能" and "有可能" are interchangeable, but "可能" is the original Chinese and "有可能" is Westernised Chinese grammar.