Is any card available which can be used to see tourist places by bus/metro in Xian?

Yes! You can ride on subway lines and buses by using the public transportation card. For instance, subway line 2 can take you to Bell Tower, and bus line 5, 19, and 21 can take you to the Southern Square of Big Wild Goose Pagoda.

You can buy the card at the customer service centers of subway stations, bus companies, local commercial bank, China Construction Bank.
By the way, you need to pay CNY 50 (with CNY 18 as the deposit, and CNY 32 as the initial stored value) to purchase the transportation card.