How does Xoom process a transfer from the US to a foreign bank account such as one in Mexico?

Xoom will process your card (i.e. take money from your card), do an overseas transfer (usually batched processed, via SWIFT transfer) at the end of the day to settle with all the banks overseas with whom they deal.

If Xoom deals with your bank in Mexico, your bank will simply classify it as an inward remittance and credit your account (at the exchange rate that Xoom has pre-arranged with your bank).