You can use the python's requests library for fetching data. To display values in XML, you have to make columns in the table compatible for storing your data returned from the link. If you need to display with website templates then no need of columns in DB, but you have to write the template. Goto Odoo's Documentation is well documented and some external things, you have to find from the correct sources.

eg:

def get_ip_address(self):
    r = requests.post("http://ifconfig.me")
    data = r.json()
    return data

In this example, I am getting the IP address of PC using the API and it returns some data as text. You have to process the return data according to your API documentation.