code: public Number getCV()
     {
        Number variance = (Number) getCachedValue(AssignmentField.CV);
        if (variance == null)
        {
           variance = Double.valueOf(NumberHelper.getDouble(getBCWP()) - NumberHelper.getDouble(getACWP()));
           set(AssignmentField.CV, variance);
        }
        return (variance);
     }
docs: The CV (earned value cost variance) field shows the difference between
  how much it should have cost to achieve the current level of completion
  on the task, and how much it has actually cost to achieve the current
  level of completion up to the status date or today's date.
  How Calculated   CV is the difference between BCWP
  (budgeted cost of work performed) and ACWP
  (actual cost of work performed). Microsoft Project calculates
  the CV as follows: CV = BCWP - ACWP
  
  @return sum of earned value cost variance