Bumping old post:
Having Kosovar stamps is not a Problem: however, if using a passport, you cannot enter Kosovo from Montenegro or Macedonia, or by air, and then continue to Serbia - you can only travel from Serbia to Kosovo and then back.
To be exact: on the way from Kosovo to Serbia, there will be one Kosovar Exit check, and then a Serbian entry check. The Serbians will then refuse you entry if you don't have a current Serbian entry stamp (which is only possible if you entered Serbia from a third Country and then went to Kosovo)
There is one way around this rule, though it's only possible for (most) European nationals: using an ID Card instead of a passport. They cannot be stamped, so the Serbs cannot prove how you entered Kosovo.
However, the ID Card must be one accepted for entry to Serbia, i.e. from an EU Country, Bosnia, Macedonia, Montenegro or Switzerland (and of course, Serbia or Kosovo)
A number of ID Cards valid for Serbia are not valid for Kosovo (French, Austrian, Polish etc.) so then you must be careful to use the passport for the Kosovar check, and the ID Card for the Serbian check