First, get the source (if it is not already on your machine).
You can type "which xv" to let you know where it is installed (if it is not in your path).
Note: you have to modify the file xv.h somewhat to take into account some inconsistencies with stdio.h ...
To take a screen shot, start xv, then right click on the splash screen.
The xv control panel then appears.
Click the "Grab" button, and hide the xv panels (if you want). The left mouse grabs the active window, and the middle mouse grabs a user defined region. Note: if your mouse only has two buttons, pressing both simultaneously emulates the middle button.
An example screen shot appears below
xv also contains sophisticated image editing feature, for example the following colormap editor:
Possible uses for screen shots: