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As CKE says, it's the original (ancient Greek) spelling. As for the sound, say the F sound and then the V sound: your lips, teeth and tongue are in the same place for both, and the only difference is that the air from your lungs moves more powerfully for the V sound: F is unvoiced or voiceless, and V is voiced. Vowels are voiced, they have to be, so when a consonant is sandwiched between two vowels, it easily becomes voiced itself. We're going back millennia here, so the present pronunciation of Stephen is very traditional. But, people who don't know this or don't care will use Steven because it seems easier or more obvious to them. I prefer Stephen, partly for its historical accuracy and partly because Steven looks like dumbing down and also can't really have a female form. And I really like the name Stephanie!

We would have to get input from an expert on medieval English to explain this, but the pronunciation of Stephen with a "v" sound in English goes back at least to the year 1279, because the original spelling used the "ph" as in the Greek name Stephanos that the name is derived from, but there are examples of the name spelled with a "v" from written records in England in 1279.