  The origin of the high temperature superconductivity in the iron-based
superconductors remains elusive after being extensively studied for more than a
decade. Determination of the pairing symmetry is essential in understanding the
superconductivity mechanism. In the iron-based superconductors that have hole
pockets around the Brillouin zone center and electron pockets around the zone
corners, the pairing symmetry is generally considered to be s$_\pm$, endowing a
sign change in the superconducting gap between the hole and electron pockets.
For the iron-based superconductors with only hole pockets, however, a couple of
pairing scenarios have been proposed but the exact symmetry is still highly
controversial. Here we report our determination of the pairing symmetry in
KFe$_2$As$_2$ which is a prototypical iron-based superconductor with hole
pockets both around the zone center and around the zone corners. By taking
laser-based angle resolved photoemission measurements with super-high
resolution and at ultra-low temperature, we have precisely determined the
superconducting gap distribution and identified the locations of the gap nodes
on all the Fermi surface around the zone center and the zone corners. The
complete superconducting gap structure, in combination with the observation of
the spin resonance in neutron scattering, provides strong evidence on the
s$_\pm$ pairing symmetry in KFe$_2$As$_2$ with a gap sign reversal between the
hole pockets around the zone center and the hole pockets around the zone
corners. These results unify the pairing symmetry in the hole-doped iron-based
superconductors and point to the spin fluctuation as the pairing glue in
generating superconductivity.
