  X-ray observations play a crucial role in understanding the emission
mechanism and relevant physical phenomena of magnetars. We report X-ray
observations of a young magnetar SGR 1900+14 made in 2016, which is famous for
a giant flare in 1998 August. Simultaneous observations were conducted with
XMM-Newton and NuSTAR on 2016 October 20 with 23 and 123 ks exposures,
respectively. The NuSTAR hard X-ray coverage enabled us to detect the source up
to 70 keV. The 1-10 keV and 15-60 keV fluxes were $3.11(3)\times10^{-12}\;{\rm
erg\;s^{-1}\;cm^{-2}}$ and $6.8(3)\times10^{-12}\;{\rm erg\;s^{-1}\;cm^{-2}}$,
respectively. The 1-70 keV spectra were well fitted by a blackbody plus
power-law model with a surface temperature of $kT=0.52(2)\;{\rm keV}$, a photon
index of the hard power-law of $\Gamma=1.21(6)$, and a column density of
$N_{\rm H}=1.96(11)\times10^{22}\;{\rm cm^{-2}}$. Compared with previous
observations with Suzaku in 2006 and 2009, the 1-10 keV flux showed a decrease
by 25-40%, while the spectral shape did not show any significant change with
differences of $kT$ and $N_{\rm H}$ being within 10% of each other. Through
timing analysis, we found that the rotation period of SGR 1900+14 on 2016
October 20 was $5.22669(3)\;{\rm s}$. The long-term evolution of the rotation
period shows a monotonic decrease in the spin-down rate $\dot{P}$ lasting for
more than 15 years. We also found a characteristic behavior of the hard-tail
power-law component of SGR 1900+14. The energy-dependent pulse profiles vary in
morphology with a boundary of 10 keV. The phase-resolved spectra show the
differences between photon indices ($\Gamma=1.02$-$1.44$) as a function of the
pulse phase. Furthermore, the photon index is positively correlated with the
X-ray flux of the hard power-law component, which could not be resolved by the
previous hard X-ray observations.
