Rule 2-3-4c : "...To push, PULL' or ward off an opponent when the ball is loose IF he may legally touch or possess the ball." This backs up GoodScout's post. R can take houlding penalty either as tack-on or previouse spot.
2-3-4c says that K is allowed to do it though.
This is definitely an "I'd have to see it" call though. On a free kick, if K knocks the receiver down while running towards the loose ball (grounded, having gone at least 10 yards), I'd say that's legal under 2-3-4c. If K tackles R while moving away from the ball so one of K's teammates can recover the kick... then I'd say that could be holding.
This is also me probably remembering a very old rule, and it may no longer apply, but is K legally touching the ball if such contact would be "first touching"? Meaning, on a scrimmage kick, if R hasn't touched the ball first, K would be on a tighter leash as to what would constitute holding when going after the loose grounded kick.
I only remember this much older rule from a NFL Films video from the 80's where they had an odd play and asked players/coaches what happened - and I think Don Shula (or Marty Schottenheimer?) was the only one to know what happened: 4th down with 2 seconds left in the half, K punts the ball away, R was going for the block and didn't put a returner back deep. Punt wasn't blocked, and K downs the ball at the R10 as time expired for the half. At the time, the half couldn't end on any "violation of the rules" to which first touching was considered a violation (albeit one without a yardage penalty) and the period was extended by an untimed down. This is obviously no longer the rule, but it is something that has stuck with me.
