My definition of patch writing would be someone copying down part of someone elses work, not in its entirety, but changing only part of the content from the source.
Paraphrasing to me is basically giving a generalization of what someone has said. Instead of picking out every detail, you focus on the main points, change the words around into your own, and re-say what has already been said.
Plagiarism is copying down someone elses work, word for word, and not giving them credit in your bibliography/citation, or putting what they said into quotes. This is the worst possible thing you could do.