In American English (and perhaps other dialects), Italian 'ciao' is an allophone with English 'chow'. (Really two allophones, 'ch' and 'ow'). Most Americans won't be able to hear a difference.
"ciao" and "chow" are homophones, not allophones. Allophones are different phonetic representations of the same phoneme found in complementary distribution. For example, [t] and [th] and [ɾ] and [ʔ] are allophones of /t/ (the first in 'cat', the second in 'tack', and the third and fourth in 'cutter' , depending on your regional variant. Also, the initial phoneme in Italian "ciao" is for all purposes identical to the one in English "chow" (the vowels are a bit different) - it is written as: /tʃ/