In some words of Iranian origin, a و vāv preceded by a خ [x] and followed by a long vowel is silenced in Modern Persian: Middle Persian خواب xwāb ‘sleep; dream’, is now pronounced xāb; خویش ~ Middle Persian xwēš ‘self, one’s own self’, is now pronounced xīš, etc.

In words of earlier Iranian origin with xwa- (short /a/) segment, normally the و is shortened as [o]: خود Middle Persian: xwad ‘self,’ Modern Persian: xod; خوش Middle Persian: xwaš ‘pleasant, sweet, nice,’ Modern Persian: xoš.

Note that in dialects more conservative than Standard Persian the inclusion of the [w] in the articulation of this class of words has been retained, as in Dari (of Afghanistan) xwaš, xwāhar ‘sister.’

The omission of the و only pertains to words of Persian origin; in the loan words (most probably from Arabic or Turkish, because of x), و is pronounced as written: خواصّ ‘the elite’ is pronounced in Persian as xavās (Arabic xawāss, with Persian /v/ for Arabic /w/).

Here is a partial list of the most common words in this class:

خواب
xāb
‘sleep, asleep; dream; the nap (of a cloth)’

خوابیدن
xābidan
infinitive: ‘to sleep, lie down’

خواب دیدن
xāb didan
comparative verb: ‘to dream’; literally, ‘to see a dream’

خواجه
xāje
‘a man of distinction; a venerable or a wealthy man; master/owner; also, eunuch’

خوار
xār
‘despised, contemptible, abject’

خوار
xār
‘man-eating’

آدمخوار
ādamxār
adjective suffix: ‘-eater/-drinker; -eating/-drinking’

خوارزم
xāraz
‘Chorezmia’ (a region in ancient Iran)

خواست
xāst
noun: ‘wish, will’ (also 3rd singular simple past tense of the verb)

خواستن
xāstan
infinitive: ‘to wish, desire, want’

خواستگار
xāstgār
‘a gentleman caller; a gentleman seeking a young lady’s hand’

خوانا
xānā
‘legible’

خواندن
xāndan
infinitive: ‘to read; to sing; to call (upon), to convoke’

خواهر
xāhar
‘sister; (slang) a (male) homosexual’

خواهش
xāheš
noun: ‘a request, desire; begging; asking for…’