The last type of speech is the extemporaneous. The dictionary definition states: it is made up or done without preparation. Most people consider extemporaneous or impromptu speaking as interchangeable. In the world of public speaking it is the opposite. Experience speakers will tell you that an extemporaneous speech is thoroughly planned but not memorized.
This allows a speaker when confronted with an unplanned event such as the projector failing, to deal with the event and then proceed without any problem. If during your speech an audience member raises their hand, you can answer their question and then continue.
An extemporaneous speech is carefully planned and prepared. This is also followed by extensive practice. Since the speaker is not memorizing the speech, they may try different words or phrases to capture the desired effect. During the speech the speaker may have practice one phrase or gesture then decide to use a different one.
Using a memorized or impromptu or extemporaneous speech is dependent on circumstances and personal style. The key is to experiment and find what works best.