No, Pithing Needle doesn't target as implicated by the non-existence of the word "target". Magic wording is very precise and you only target if a card says so. So, no, you can't use Sliver Overlord's abilities when it wsa named with pithing needle. Hexproof does nothing in this case.

What can Pithing Needle target?

2005/6/1 Pithing Needle affects cards regardless of what zone they're in. This includes cards in hand, cards in the graveyard, and exiled cards.

Does Pithing Needle stop Valakut?

RDW is a horrible match up for Valakut. Also try to keep them at one or zero green sources, although land destruction in general is too slow / easy to play around. Pithing Needle shuts down Terramorphic Expanse, Evolving Wilds, and Expedition Map.