If you check out any of the PBusardo videos and fast forward to the section where he tests the voltage output. All of the low end vv devices, the Vamo, the SVD, Zmax etc are using PWM or Pulse Width Modulation without filters.PBusardo says that you can tell because when you vape on these devices on certain atomizers they sound like a rattlesnake.
The Provari, The Itaste MVP, the DNA20D Mods all have filters or are using different technology and they are outputting a straight voltage amount.
You may ask what is the difference. Well the PWM has a fluctuation in voltage that is very large and very quick, some say that you can't detect it. Some even like it better. Some say it make the atty sound like a rattlesnake and is harsher.
For example on a Vamo if you set the voltage to 4V the voltage will fluctuate between around 7v and 3v the duty cycle will be short and the ending mean is the 4v that you've set.
A Picture is worth a thousand words, I captured the top two pictures from PBusardos You Tube Review of the Vamo and the bottom from his Youtube Video of the MVP.
Picture 1 showing the 33hz chip that the Vamos Uses
Picture 2 Showing the Voltage Wave fluctuation of a Vamo
Picture 3 Showing the Voltage Wave Fluctuation of a MVP or Provari or DNA20D