In the past 10 years the number of hostels has grown from about five to almost 150, giving you an endless variety to choose from.
Most of Medellin's hostels are located in El Poblado particularly around Parque Lleras, Provenza and Parque Poblado. The reason for this is simple, it's not only the safest area in town it's also the main dining and entertainment area. There are tons of restuarants, you could potentially try a different one for lunch and dinner everyday for a month and never bore of the variety. When you want to have a night out Parque Lleras is a short walking away and easy staggering distance back.
Hostels come in all sizes and varieties, from the high-energy party hostel through to the tranquil, relaxed hostel. It's always a good idea to check Tripadvisor and/or Hostelworld reviews before booking and if in doubt book a couple of days and see how it goes, you can always extend.