Are racist/xenophobic opinions sufficient to revoke tenure?
There are four common reasons for tenured professors to be fired:


Incompetence. Used here in the more general sense of the word, it means that if the tenured professor is no longer able to carry out his or her duties - for example if he or she becomes vegetative after an accident - then they can be fired.
Negligence. If the professor is able to do the work, but does not do it, then it could qualify as negligence. For example if the professor decides to go on vacation during semester, thereby leaving the students without instruction, he/she could be fired.
Immoral conduct. If the professor sexually harasses a student, commits a (serious) crime like robbery, or violates academic norms (e.g. plagiarism), he/she could be fired.
Financial exigency. If the university runs out of money for whatever reason, it can fire professors.


Holding unpopular opinions don't come under any of these four reasons. The most one could argue is that it's immoral, but that would be hard to defend since not everyone agrees that racist/xenophobic behavior is wrong. After all it was pretty common to have racist/xenophobic behavior in the past or even today.

But: it has happened in the past. Professor Edward Ross of Stanford University was effectively fired by Jane Stanford, one of Stanford's founders, back in 1900. Ross expressed some violently racist opinions of Asians. His firing was controversial, and he was defended by many academics who viewed the dismissal as arising from political reasons and therefore a violation of academic freedom. You can read more about it in articles like this one. I would venture that if Ross lived today he'd be less likely to be forced out, but it'd be within the realm of possibility, and regardless of whether he's fired or not it would still be controversial.