As everyone knows from reading the ADOM manual, DV is an abbreviation for Defensive Value and PV is an abbreviation for Protection Value. The former determines how hard it is for an opponent to hit the PC; the latter determines how much damage is absorbed by a PC's armor if a monster makes a successful hit. In recent gammas of ADOM, there has been a tendency for monsters to score more critical hits which bypass PV. Some monsters always bypass PV, notably claw bugs, which make them very annoying opponents for low level characters. Note also that non-artifact armor can be damaged by monster attacks, lowering its Protection Value especially and eventually destroying it.
So the question arises, which is better, a high DV or a high PV? As with the question of whether to specialize in one weapon or become proficient with many weapons, it is largely a matter of personal preference and style. There are successful players who use both approaches. In practice, it is possible to raise DV to the point where none other than the very highest level monsters can hit the PC. A DV over 100 means very few monsters will hit the PC successfully. However, a DV this high is not available to any but the very high level characters that specifically pursue it. Likewise, a PV over 100 means few monsters can hurt the PC, since the vast majority do not deal over 100 points of damage in one round. Bear in mind, however: 1) the possibility of critical hits and 2) the possibility of running into something very nasty, such as balors, Greater Molochs and the Emperor Moloch which certainly do deal over 100 points of damage per round.
A general rule of thumb? Add up the DV and PV bonuses. A [-3, +5] iron chain mail is about as good as a [+0, +2] leather armor (and the latter is lighter and won't rust). Few non-artifact armors can compete with an average eternium plate mail at [-1, +13]. Dragon scale mails averaging [-2, +11] are also extremely desirable armors since they grant immunity to the element corresponding to the dragon the armor is derived from. On the DV side, few non-artifact items can compare to eternium tower shields averaging [+16, +6]. Dragon scale mails and eternium tower shields are so powerful that many players will use a wish on these items.
The debate over DV versus PV is one that will never be settled, which is as it should be. ;-)