Few definations which I came across-
An invention is a new composition, device, or process. An invention may be derived from a pre-existing model or idea, or it could be independently conceived in which case it may be a radical breakthrough.
Discovery can lead to invention. A specific, tangible application of discovery. What is discovered already existed; what is invented did not exist before. Invention results from creativity applied to discovery.
Discovery can also lead to innovation. Innovation is based on something already existing. Innovation is the novel application of something. It can be an idea, or a tangible thing . . . again, commercial or not. Much of the time innovation is the extension of invention. Innovation is also a product of creative behavior.
If you didnt get any of above sentences, lets make it simple.
Discovery is of something that is already present but not in knowledge of humanity whereas innovation is creation of something which doesn’t exist.
Thus, Invention: Creation of a new item. Discovery: Find something new that was otherwise already present, but not known to be present. Innovation: This is a change in the thought process for doing something or “new stuff that is made useful”.
All what this topic wonder around is understandability of concepts and creative mind.