Tabitha can be Tabs, Tab, Taba (my late best friend used Taba), Bit, Bitsy, Tib, Tibs, Tibby. My favorites are Tib/Tibs/Tibby.
Xavier could be Xavi, Xave/Zave, or just X. X is a great, rare initial. Like Q, I really enjoy it as a nickname.
If you're nickname people then, even for names that don't have obvious nicknames, a nickname will evolve over time. Tabitha and Xavier are both great with Lennon. I say don't let the nickname thing hold you back.