The term boutique hostel is so new that you can’t even find it in Wikipedia. Go on. Try it. In fact, boutique hostel may even be an oxymoron, and I’m sure backpackers of the world will not be taking a particular shine to this phrase.
Nonetheless, something that seems to fall between a regular hostel, B&B, guesthouse, and boutique hotel has cropped up. Of course, it’s all marketing — they’re really just “very nice hostels.”