The majority of English speakers do not exactly pronounce a "w" at the end of a syllable like "show" or "fellow". There is a written w, and a consonantal "w" sound did indeed exist in these words historically. However, because of the historic tow-toe merger, the sound in these words is now identical for speakers of most dialects to the "long o" sound used in words not written with w like "go" or "hello". The exact sound used for this "long o" varies among speakers.
However, there is something that may sound like a "w" in many speakers' pronunciation of all of these words (both words like tow and those like toe). Many speakers use a diphthong in these words, which is a sort of gliding vowel that moves between two vowel sounds within the same syllable. We write /oʊ/ to represent a sound that starts out sounding like /o/, and moves towards an /u/- or /w/-like sound. The convention is to represent this with the symbol /ʊ/ instead of /u/, because in English the symbol /u/ is used for a long, pure /u/ sound, and /ʊ/ is used for a shorter, less distinct counterpart to /u/. (The symbol /w/ is generally not used because these words are not considered to end in a consonant.) Using a diphthong for the "long o" sound is part of the most standard dialects of both the United States and England, and most phonemic transcriptions of English are based on the standardized pronunciations used in these two countries.