"pkd" process on El Capitan
The manpage says:

NAME
 pkd -- management and supervision daemon for plug-in services

SYNOPSIS
 pkd [-v]

DESCRIPTION
 pkd manages plug-ins for the PlugInKit subsystem. It is a classic launch
 agent managed by launchd.


Sounds pretty safe to kill it off, by opening a terminal and running:

launchctl stop com.apple.pluginkit.pkd


It should restart itself whenever it's required (possibly immediately).