Is the UTXO associated with output and Tx id?
Yes, you're exactly right, each UTXO is basically just a tuple (txid, output_index). Then when that output is going to be spent, you refer to it by listing the tuple as an input.
Essentially, that's why they're called "Unspent Transaction Outputs" (UTXOs), not "Unspent Transactions".
For example, look at this random transaction output I pulled of of the block chain:
txid: 564bb87efc20751dde182b678e848ceaa75ac54fd2eab33808842f40e953f7e6
output: 1
scriptPubKey: 76a9148ab215e3d638357551f5e6e7e3384b56bb355a6c88ac
(Note that 1 is the index, meaning it's actually the second output)
Then when the transaction is spent, the input references this previously unspent transaction output.
ID of spending TX: 57149f629790e61a5fbab9e27041858ada6348aa17323c15b9da9659b90717bb
input:
"txid" : "564bb87efc20751dde182b678e848ceaa75ac54fd2eab33808842f40e953f7e6",
"vout" : 1,
"scriptSig": 48304502205b6d49f0e473e5b6134c9ba55dcfdd9e010a226fb29f89e9f4ff1a2b568a1d4c022100af290372ed889ffff948662d7a5b1f2d6eb29dbdcad260ab53f99aba6b6e46e50121020f4db25f5bd35cb2c541ecdbcdb0e7c0be890cb4e25895612f8a1cbb32216ff0