More comprehensive answer: All 2-letter top-level domains (TLDs) starting with X (that is: .xa through .xz) are reserved as "alpha-2 user-assigned codes" that can be given as either TLDs in private networks (not accessible from the general internet) or as temporary "placeholder" TLDs for domain namespaces that are expected to later get a permanent TLD but for which the immediate status is unclear or for which that process is pending (among other uses).
.xk is sometimes used for Kosovo under this last auspice, but is explicitly non-permanent, and is not guaranteed to work in all cases and from all requesting origins (e.g.: depending on your ISP, you may not be able to reliably resolve/connect to a .xk site).
Slightly longer answer: It is sometimes used as such, but is only temporary and unofficial.
More comprehensive answer: All 2-letter top-level domains (TLDs) starting with X (that is: .xa through .xz) are reserved as "alpha-2 user-assigned codes" that can be given as either TLDs in private networks (not accessible from the general internet) or as temporary "placeholder" TLDs for domain namespaces that are expected to later get a permanent TLD but for which the immediate status is unclear or for which that process is pending (among other uses).
.xk is sometimes used for Kosovo under this last auspice, but is explicitly non-permanent, and is not guaranteed to work in all cases and from all requesting origins (e.g.: depending on your ISP, you may not be able to reliably resolve/connect to a .xk site).