How MS Project calculates Cost Variance (CV) for an ongoing or finished task?

Actually CV = EV-Ac as EV is Budget cost of performed work, AC is Actual cost of work performed.
MS Project Considers EV = PV if it is not entered independantly where PV is Budget cost of Planned work.
Accordingly you shall enter PV for each activity and AC as well.
then run model so CV will be calculated.

If you had saved the Baseline before updates and then run the variance report, the system will do the calculation and present you CV.
Primavera is superior in terms of Earned value analysis.
MSP is fit for simple Task Management/Scheduling.