According to the [Bulgarian Ministry of Foreign Affairs][1], as a Bulgarian citizen you need a valid Bulgarian passport to enter Kosovo.

For citizens from other EU countries it is analogical. They need a valid passport of their home country. I have checked the sites of several ministries of foreign affairs. They all say the same thing. 

P.S. [This information][2] might be relevant too, although it is not directly linked to your question.


  [1]: http://www.mfa.bg/120/findanembassy/0/179
  [2]: http://travel.stackexchange.com/a/4501/3470