Defense Value is a static defense system that replaces rolled parries and dodges with a set number that must be beaten to successfully attack. Defense Value equals half the pool (rounded up for Exalted) of the defense being applies.
Your DV is applied to an attack automatically, you just select if you want to use your Dodge DV or your Parry DV and which weapon's Parry DV you want to use. You can apply your DV to every single attack made against you freely.
Charms, for the most part, work normally. Here's a breakdown of how different charms work with DV instead of rolled defense.
If you have an effect or item, like Hearthstone Bracers, that add dice to defense rolls, simply add them to the pool from which your DV is calculated. Stuff that adds automatic successes to defense rolls adds 1 directly to your DV per success added. Durations remain the same.
Your DV is lowered by 1 for every attack made in a turn as well as by 1 for every action taken beyond the first. These don't stack. If you take three attacks in a turn your DV is -3 and if you take three non-attack actions and then an attack, your DV is -4.