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Hello, actually a private coin is a coin that we can retrace from address to address like we can with BTC for example, it means you can't really know what is on the address with a blockexplorer,
PIVX is already in the market and you can buy it here : https://www.cryptopia.co.nz/Register?referrer=membercrypto
Hope I've answered to your questions

The Private coin is a category of coins which focus to maintain your privacy while you do your transactions. Some examples are Monero, Zcash, PIVX, Komodo, Zcoin, NEVcoin, ZenCash, Verge, Dash. Correct me if i am wrong.

And POSW, future STAKENET. Next month rebrand and swap coins. POSW is a fork from PIVX with masternodes as well. Better option