At a first glance, one can understand your question as the difference subtraction of PV from EV, which is the schedule variance. SV = EV - PV.
To answer your question, as what is the difference between the two, EV means how much value are we really getting from this project at this point in time, while PV is how much value we should be getting from the project as originally planned.
So when will the two be equal?
Since EV = BAC x (% of the actual work done)
and PV = BAC x (% of the planned work done)
This means that if EV = PV then BAC x (% of the actual work done) = BAC x (% of the planned work done), this means that (by elimination), we have that if EV = PV then (% of the actual work done) = % of the planned work done), this means that the two are equal when the percentage of the work done is exactly equal to the percentage of the work planned, which means that the project is progressing as per the original schedule.
Here's an important formula related to EV and PV:
SPI (Schedule Performance Index) = EV/PV . If SPI is bigger than one, then the project is ahead of schedule, if it's lower than 1, then it's behind schedule.