The CV is a statistical parameter that describes variance of data.
For each group of data (e.g. wells of plates) CVs are calculated for each selected subset (e.g. control wells of a plate).
CV = sd(x[subset] / mean(x[subset]) * 100
or
CV = mad(x[subset] / median(x[subset]) * 100
Literature: Zhang, J. et al., J Biomol Screen 1999; 4; 67
Wikipedia: Coefficient of variation