The unit of electric potential difference is:

Electric potential difference is defined as the work done in bringing unit charge from one point to another. 

$\therefore \mathrm{Potential} \mathrm{difference} \mathrm{or} \mathrm{voltage}= \frac{\mathrm{Work} \mathrm{done}}{\mathrm{charge}}$

$\mathrm{SI} \mathrm{unit} \mathrm{of} \mathrm{potential} \mathrm{difference}: \frac{\mathrm{SI} \mathrm{unit} \mathrm{of} \mathrm{work} \mathrm{done}}{\mathrm{SI} \mathrm{unit} \mathrm{of} \mathrm{charge}}$
$\therefore \mathrm{SI} \mathrm{unit} \mathrm{of} \mathrm{potential} \mathrm{difference}: \frac{\mathrm{joule} }{\mathrm{coulomb}}$

SI unit of potential difference is also denoted by volt, 

$\therefore 1 \mathrm{volt} = 1\frac{ \mathrm{joule}}{\mathrm{coulomb}}$

Therefore, the SI unit of potential difference is $\frac{\mathrm{J}}{\mathrm{C}}$.