I write this article — painfully — on August 11th, 2019 as I have just had to go through this. This is the most up to date information on travel to Serbia from Kosovo.
Serbia DO NOT RECOGNISE KOSOVO. Therefore you cannot ENTER Kosovo and then go to Serbia, as you will not have a SERBIAN ENTRY STAMP. Read that very carefully.
Serbia – Kosovo – Serbia
This is the best way to visit Kosovo without hassle. You enter Serbia, go to Kosovo, leave through Serbia. Therefore, you have never “left” Serbia. Still following?
Can I travel directly from Kosovo to Serbia?
OK – here is where it gets complicated. You can travel from Kosovo to Serbia if you have an EU national ID card, but you will not have an entry stamp for Serbia, so when you leave to a third country you risk a fine, or worse. Also according to my friends in Kosovo, tourists are routinely sent back.
How can I travel legally from Kosovo to Serbia?
Sadly, you need to go via a third country, which will mean Montenegro, Bosnia, or Macedonia. Macedonia is only an hour from Pristina, and about the same from Prizren (although the buses are much less regular).
The next pain in the ass thing to do is take a bus to Nis (5-6 hours) and then transfer for a bus to Belgrade, which takes another 3 hours or so. Essentially you need to drive around Kosovo.
As stated it would be possible to do this via Montenegro, but this would add even more to the journey.
One sincerely hopes that everyone grows up, moves on and things get easier, BUT this is the Balkans after all!