To get the master private key of HD wallet How can I get the master private key of HD wallet bitcoin core satoshi client 0.14.2? Any command for that  on bitcoin core console? Currently I am on Testnet3.
go to help-> debug window -> console

Enter:

walletpassphrase "your walletpassphrase here" 60, to unlock the wallet if its encrypted.

dumpwallet  "filename", will crate a plain text file in same directory where your wallet .dat file is form which you can get your xprv.  

walletlock, to lock your wallet  

Don't forger to erase the file after you get the info. Also if u change your wallet password the xpri will change.