XV0 and V0 - what does it stand for?
You can connect those pins to 3.3v if not mentioned. Anything more than 3.3v without mentioned can be disastrous. The V0, XV0,VG is used to power segment drivers and common circuit drivers inside the LCD. Without powering thses pins,you can't able to see images in LCD but it still accepts the commands and works fine. The Vdd and Vss is to power the microcontroller inside the LCD to work. The V0,XV0and VG is used to power the display to show images.   You can see from the picture that you have COM and SEG lines.These will drive the Liquid crystals inside LCD which inturns shows you as dots. To get that image ,you need to provide separate power right, that is what the purpose of V0,VG and XV0 pins. In datasheet they also have a capacitor between then and ground for avoiding surge voltages and noise from power supply.  You can see that power for display , they provided these pins. I also found the necessary supply that has to be provided from the LCD driver datasheet  Kindly refer ST7567.pdf to know more about driving the display and power supply requirements.