Here are some tips for all of you that want to level up as fast as possible:
Find the best game modeThere are several different game modes in the multiplayer of Modern Warfare, but either Search & Destroy or Domination provides the best XP per hour by far. These two are very different, and we suggest going for Domination if you just want to chill and kill other players while getting a good amount of XP, or play Search & Destroy if you really want to focus on the objective. The latter one is amazing in terms of XP gained if you are able to play well, but it might be worse than Domination if you don't really play all that well. So try both and stick with the one that suits your skill level the best.
Another alternative is to join a Ground War game. In here, hop in a tanks and start grinding XP. Most players find this to be slightly faster than the two game modes mentioned above, but it's also way more grindy and much more repetitive.
Complete MissionsIf you head to the Barracks, you can see your Daily Missions as well as other Missions that you can complete. These give you XP, as well as some other cool rewards, so they are often worth completing if you want to level up as fast as possible in Call of Duty: Modern Warfare. Always make sure that you actually head to the Barracks to check for new missions, because you will need to activate them before you get progress towards them.
Keep in mind that not all Missions are worth doing, so if they seem like a huge waste of time, you might be better off getting that XP from other sources. However, you might get lucky and get a Mission that will want you to do something that you might be doing even without the Mission, which means it's just free XP for you.
Some players are having a bug where you don't get progress towards the Daily Mission, but this is just a visual bug, so you do actually get progress, it just doesn't show it in the game. Just restart your game to fix the bug.