Those Hostnames and IP's are most likely Spam Bots. However, they could be Probes gathering information on your site and/or server to start an attack on your website/server.
I searched that xlhost.com site, and its a Dedicated Server host, they sell dedicated servers and Virtual Private Servers. So its probably someone that is using their services and servers to gather information on your site for an attack (Spam Attack, Bruteforce, DDoS, SQL Injection, etc...). Just Blacklist those IP's and/or the xlhost.com domain hostname.