PCI Express (PCIe) was the successor to PCI and is an entirely different and incompatible interface.
You can get bridges but they typically require you to rig something up to support the card rather than straightforward being plug-in solutions.
Having said that, if I recall correctly it was only the first generation of X-fi models (circa 2006) that were PCI, all of the later ones were PCI Express. And PCI is old enough that in recent years it's not uncommon to see PCI Express being referred to as just PCI.
You don't give the specific X-fi model (I don't remember there ever being a plain X-fi model) but it's possible it's actually PCI-Express rather than PCI.
The connectors are easy enough to tell apart, especially the shorter PCI-E connectors typically used on sound cards so if you post an image of it then we could say for sure.