As a delegate you are the official representative of an antenna. You have the right and obligation to take part in all plenary meetings of the Agora. Also you have to participate in committees (Prytania) about certain topics.
As a visitor you have no obligations at the Agora, but also no voting rights. You can decide freely in which event you want to participate. Additionally to the Delegate meetings there are many different interesting progress meetings and workshops. Also you have a little bit more time to sleep and explore the surrounding area.
The Envoy represents a European body of AEGEE (for example Working Group, CD). You will probably speak a lot on stage to present the current progress of your body to the Agora. Envoys also don't have voting rights.