Welcome to the forum al, you will find that Direct Vent and Power Vent can be used interchangeably in a most confusing manor.
I have provided the generally accepted descriptions for each. I'm sure others will chime in to provide their particular views.
The Power Vent is a vent unit that mounts on the side of the building and pulls the flue gas out, there will be a barometric damper near the stove on this type installation to regulate draft and the fan speed is usually controlled by a rheostat to prevent excess heat from being lost out the stack. This type unit is available with the Leisure Line stoves as well as Alaska. You can purchase a SWG 4HDS power venter and adapt it to almost any kind of stoker with the right instructions. Jerry at Leisure Line owns the UL listing for use of this type of vent on a coal stove.
The Direct Vent is a unit mounted on the stove (bottom rear) that pulls gas out of the stove and sends it out a 4" pressurized pipe. This type unit is available on the Keystokers and Alaska stoves, and can use a barometric damper before it or a calibrated leak to ensure proper draft. The disadvantage of this type is that the pipe is under pressure in the house and has to be sealed really well or you will get CO around it.
The Harman Direct Vent (DVC 500 stoker)is a different type that actually pulls outside air as part of the process and can be found on the Harmon website.
As far as reliability, with proper maintenance the Power Vents and Direct Vents can last for up to 5 years without replacing parts. I personally have been using a SWG 4HDs on my stoves (have had it hooked to an Alaska, Harmon and Keystoker) in the house for 1.5 years.
Whichever type you use should be rated by the manufacturer for your stove and be checked with a manometer or draft meter for proper operation. I us a manometer permanently hooked up to monitor the rheostat position and verify sufficient draft.