Plain old chords are three-noters: triads. The V chord doesn't have to be a 7th, in fact sometimes shouldn't be a 7th. The latest Amazing Grace lesson I did has a couple of V chords that weren't 7ths because it didn't sound appropriate. But if you are going add that 4th note to the chord, then it's the flat 7 that goes with the V chord, the (major) seven for the I and IV ... unless of course you're in the Blues/Jazz genre where they can all be sevenths.
The reason they are different forms of sevenths is that the mother scale (of the I chord) is determining the notes in each chord. The asymmetrical way they fall (TTsTTTs) makes is so they all have a major 3rd, making them all major, but when it comes to adding the next note -- a 7, since the formula is every odd note from the scale -- two of them fall as major sevenths, one as flat seven. The best way to literally see this is on the scale clock below. Simply look at the intervals of the C, F and G chords.
I chord: C E G B -- Cmaj7
IV chord: F A C E -- Fmaj7
V chord: G B D F -- G7th
Again though, this doesn't always apply, it doesn't
have to be this way.
