Yeah, an FQDN should end with a period. But if you have intentionally put the dots (.) after the URIs then you have misinterpreted this general rule. See, what they mean by "an FQDN should end with a period" is sorta like this: subdomain.primarydomain.com - notice the dot after subdomain, that's the dot they are talking about.
You don't have to put the dot after the .com. Again, just ignore these comments if the dots were not intentional.
Now, to asnwer your question, the first link i.e. abc.xyz.com is a valid FQDN while the second ain't.
Peace.