  AT2019pev is a nuclear transient in a narrow-line Seyfert 1 galaxy at
$z=0.096$. The archival ultraviolet, optical and infrared data showed features
of both tidal disruption events (TDEs) and active galactic nuclei (AGNs), and
its nature is not fully understood. We present detailed X-ray observations of
AT2019pev taken with Swift, Chandra and NICER over 173 days of its evolution
since the first Swift XRT epoch. The X-ray luminosity increases by a factor of
five in five days from the first Swift XRT epoch to the lightcurve peak. The
lightcurve decays by a factor of ten over $\sim$75 days and then flattens with
a weak re-brightening trend at late times. The X-ray spectra show a
"harder-when-brighter" trend before peak and a "harder-when-fainter" trend
after peak, which may indicate a transition of accretion states. The archival
ground-based optical observations show similar time evolution as the X-ray
lightcurves. Beyond the seasonal limit of the ground-based observations, the
Gaia lightcurve is rising toward an equally bright or brighter peak 223 days
after the optical discovery. Combining our X-ray analysis and archival
multi-wavelength data, AT2019pev more closely resembles an AGN transient.
