What is the difference between the extemporaneous speech and impromptru speech?

The difference between extemporaneous speech and improptu speech is basically one.

Even though they can be really similar, Extemporaneous speech is when it may look like someone is speaking in front of an audience with no previous preparation, but the truth is, they had some preparation and practiced their speech before actually giving it; while Improptu speech is completely without preparation.