There are many Vietnamese consonants and vowels that do not exist in English. KH is one of them. This consonant sounds like in the middle between a K and an H.

The sound exists in the German language as in /ch/, also in Spanish and Russian languages

New Vietnamese learner’s usually confused the sound with H
This video will help you learn how to pronounce the Vietnamese consonant KH correctly
Take a look at this video

To learn more about Vietnamese Pronunciation Southern Dialect, you can take a look at here: