EVM and Schedule Variance

I am using MS Project to get Schedule Variance of a project. At this point in time I have put all the tasks associated with the project, assigned resources to the tasks (all of the resources are type 'Cost'), and create it a baseline for the entire project. If I am not mistaken, I should start tracking the progress of the proyect so MS Project can start doing its Earn Value calculations.
One of the task should have last 19 days and cost $2,500 to complete, instead it took 25 days to be completed but with the same cost. Looking at the list of dependencies I see that this particular task is not associated with any of the other task.
Here is where I don't understand what is going on: Checking the EV tables I see SV equals 0, I would think SV should be equal to a negative value, which indicates the project is behind schedule, no?

I cannot speak to how MS Project calculates EV data; however, using earned value to analyze your schedule is not advised. SV and SPI becomes 0 and 1.0 respectively as the task or project finishes. That is because the variance is measured on the yaxis, or money axis, instead of the xaxis, or time axis. So, eventually, your EV will equal your PV and so EVPV = 0 and EV/PV = 1.0.
You need to use Earned Schedule. It follows the same principles as EV; however, it uses different variables off the xaxis to calculate variances and SPIt or FV does not correct itself. So if you are late by six days, as in your example, than your SPIt will reflect that properly. SPIt would be 19/25 = .76 and will remain .76 for that task. FV = 19  25 = 6. Earned Schedule
In this picture, I show both EV and ES data. Where I circled "Finish Variance", you can see that I am measuring the gap between where we are with completed work (red line) and where it equals on the blue line (planned work) drawing a horizontal line. Where EV intersects with time is equal to actual time (AT) and where that horizontal line intersects with the blue line is earned time (ET). So you can see how this variance is measured against the Time line (xaxis) instead of the budget line or the yaxis.