There are a couple ways of doing this.
You can use Focus > Analysis > Overlay > Statistical. The primary input layer is the polygon layer; the secondary layer is the raster channel. You would need to compute the Std Dev and Mean separately, then use Field > Compute in the Attribute Manager to compute the coefficient of variation (Std Dev / Mean).
You could also use VIMAGE to compute the Std Dev and Mean.
Another formula for CV (as a percentage):
Coefficient of Variation = (Standard Deviation / Mean) * 100
Hope this helps!