This is a commonly asked question and it makes a big difference to your CV.
Basically, you should use a modern looking font that is easy to read and not too dense.
Some fonts have a smaller gap between letters and these are useful if your CV has run into an extra page by just a line or two as changing font can significantly shorten a CV. Try switching between Calibri and Arial and you’ll see what I mean.
Avoid using Times New Roman or similar fonts like Georgia as they make your CV (and you by inference!) look dated. Equally you should avoid Comic Sans like the plague!
The best thing to do is experiment with different fonts and see how they can make your CV appear until you find one you’re happy with.