PS *kysnǫti / *kvasъ PIE ū / wā (wō)
PS *xytiti / *xvatati PIE ū / wā (wō)
PS *xyrěti / *хvоrati ū / wo
? PS *ty / *tvоjь ū / wo
Is it somehow related to kʷetwóres rule?
modern Russian spelling
киснуть / квас
похитить / хватать
хиреть / хворать
ты / твой
Note:
PS *y is Old Slavic ЪI (which is a ligature of Ъ followed by I)
PS *Ъ from PIE semivowel u