http://www.ravaviner.com/2009/02/cohanim-and-kivrei-tzaddikim-graves-of.html
In Iturei Cohanim (Sivan 5766 #261),
Rav Aviner was asked, is it
permissible for a cohain to enter
Ma’arat Ha-Machpelah? He answered that
there is a dispute, but Maran Ha-Rav
Kook did not enter. Similarly, in the
weekly parashah sheet "Olam Katan"
(#169), Rav Aviner was asked, is it
permissible for a cohain to enter
Ma’arat Ha-Machpelah to recite
Selichot? He answered that there is a
dispute, but Maran Ha-Rav Kook did not
enter. In Shut She’eilat Shlomo (vol.
3 #329), Rav Aviner also writes that
although there are authorities who
permit cohanim to enter "Kivrei
Tzaddikim," since the righteous are
called "living even in their death,"
the accepted halachah is that it is
forbidden. There are also authorities
who allow cohanim to visit Maarat
Ha-Machpelah and Kever Rachel, because
they were built in a way that the
cohanim would not become impure; but
the acceptable halachah for this is
also that it is forbidden. Therefore,
we say that cohanim should not enter
"Kivrei Tzaddikim," but we can defend
the practice of those who act in this
way, especially entering Maarat
Ha-Machpelah and Kever Rachel.