Reports from the majority of backpackers I meet around town confirm that there are three youth hostels that stand out in Medellin. These are:
Kasa Kiwi Hostel, Black Sheep Hostel and Pam Tree Hostel
Kasa Kiwi and Black Sheep are located in El Poblado and within walking distance of the Parque LLeras, Exito supermarket, Berlin Bar and an abundance of other bars and restaurants.
Palm Tree Hostel is near El Diamante in an entirely different part of town which gives a more typical feel of Medellin.
Kasa Kiwi and Black Sheep have a kitchen and do grills in the evenings and have all the typical amenities such as internet access.
The easiest way to book a room at any of these youth hostels is through the following interface provided by, hostelworld.com. The prices are almost identical but personally, I think Kasa Kiwi is located in a particularly good spot on a charming street with cafes and a small fountain in front of Cafe Lolita that serves great coffee.
From what I have heard, Black Sheep Hostel is fairly lively so if that´s you´re thing, this might be the best place to go.
If you are looking to make a booking in another location in Colombia you can of course use the tool for this. It might be a good idea to reserve a place early now that the number of travelers visiting Medellin and Colombia in general is rising rapidly.