emergency landing, unscheduled landing同piority landing有咩分別?

Emergency Landing - Usually occur when an aircraft is in an emergency situation either because of aircraft technical problem (eg. fire occur inflight, engine failure) or could be pax problem (eg. emergency medical issue, pax causing problem)... An emergency landing would require them the fly to a near by airport which the airport itself will have to close a runway to allow the aircraft involve the land quickly to get the assistance they need... During an emergency landing, all other flights to/from that airport will have to suspend their take-off/landing to allow the emergency landing to land quickly safely...

Unscheduled Landing - Any flight that have landed at an airport that are not schedule to land at... The reason of unschedule landing could have many course like divertion of an aircraft during its flight due to an emgerency situation... It could also occur when the orignal destination airport of the aircraft is closed or due to weather being too bad that they will then have to divert to a nearby airport which will also be an unscheduled landing...
# An emergency landing could be a unscheduled landing as well for an airport (eg: a flight from Hong Kong-London diverted to Beijing for medical support so the aircraft will make a unscheduled landing at Beijing, and since they are in a medical emergency situation, so it would require a emgerency landing)... But an emergency landing doesnt have to be a unscheduled landing for an airport as if the emergency situation occur near their destination airport...

Piority Landing - Not as urgent as emergency landing... This usually occur when an airport is partly suspending their operations (eg: an airport is suffering from bad weather and is causing huge delay, they will give piority landing to all long-haul aircraft which may not have the information before they depart and all short-haul aircraft could stay at their orignal destination to wait for the weather to get better... This piority landing position give the airport a clear image of how many flights around and may require to divert to nearby airport when they run out of fuel...)

Hope this will give you a clear image of what Emergency Landing, Unscheduled Landing and Piority Landing are...
And in fact, there has been some confusion over the term Emergency Landing and Piority Landing before in incidents (eg. airlines with pilots not having english as their first language use to think that Piority is same as Emergency until the accident in New York when a flight from South America was running out of fuel while circling around New York waiting to land finally begin to run out of fuel, and they requested a Piority Landing instead of Emergency Landing... So they crash at the end without being able to land at New York...)... But after the incident, ICAO have given a better discrubtion of Piority Landing and Emergency Landing and in what situation these terms are to be used...