but they are more than just galvanometers then hence an added resistance to gather the PD information.So you measure Potential difference with a Galvanometer do you?You can do, if you use it to measure the current passing through a known series resistance.
They may be called voltmeters, but moving coil instruments are just sensitive galvanometers with a switchable selection of series resistances built into the case.
DVM's tend to use the resistance in an R/C network to control either a microcontroller or an oscillator the pulse train from this is counted and the time function gives the vakue of the voltage measured the display is configured to make sense of this value into the voltage reading we see, with a bit of simple circuitry it is quite easy to get other functions such as frequency reading etc, hence why a lot of DVM's are multi functional.