These words have different meanings but are sometimes pronounced the same, in some English dialects. One can pronounce both words with the second syllable sounding like ill, or one can pronounce the second syllable of hostile like aisle.
Hostel is a noun referring to an establishment similar to a hotel, but much more casual, cheaper, and often thought of as a place for young people. Some hostels, managed by youth organizations, may require that guests are students or young people, however most hostels do serve people of all ages. Hostels usually have more dormitory beds than private rooms, a shared kitchen, and a public meeting space; they are often used as homes by backpackers, expatriates, and other budget travelers.
Hostile is an adjective describing a person or animal who shows unfriendliness towards others. Hostility (the noun for hostile) can be violent and dramatic or cold and quiet. However, one would probably not call an introvert “hostile”; a hostile person actively demonstrates unfriendliness in some way—such as with facial expressions and tone of voice.