All of them may be fairly categorised into "Chinese", but history makes them different in terms of living standard, education, government, legal system and even slangs and eating habits. But how do outsiders see these people?
Undeniably both Hongkongese and Taiwanese are grouped into Chinese. Neither Hongkong's particularity as a SAR at present nor its colonial history can be taken as a proof of its independent sovereignty. Taiwanese, regardless of Taiwan's contemporary reigning independence, still has countless ties with mainland Chinese. Owing to differences in geographical location, regional custom and historical influence, otherness in culture is unavoidable.
Who's Chinese is always an interesting question. If I was born in Hong Kong, could we say we 're biologically, culturally Chinese, or simply politically Chinese? The answer is doubtless to nationalists, but by nature it may not be. Many of my Caucasian friends were born in Hong Kong and enjoy the legal status of being "hongkongese", but no one see them as Chinese.
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I don't think hongkongnese is the language they speak.... I thought it was.. Cantanese that the people from Hong Kong speaks.....
Well, although Hongkongese is only a word to describe the citizens of Hong Kong. Cantonese is the major language in Hong Kong. However, the Cantonese language in Hong Kong differs from the original Cantonese language in Canton province in China in terms of the pronunciation and word use, thus Hong Kong Cantonese or Hong Kong would be a better word to describe the language that the Hongkongese speak.
Anonymous However, the Cantonese language in Hong Kong differs from the original Cantonese language in Canton province in China in terms of the pronunciation and word use, thus Hong Kong Cantonese or Hong Kong would be a better word to describe the language that the Hongkongese speak.
That is very interesting.

Is it something like the difference between American English and British English?

Or is the difference between Cantonese and Hong Kong Cantonese much greater than the difference between

"American" and "English"?
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Sorry, I have grammatical pedantry syndrome. Please use "If I were born ..." and "no one regards them".

"Biologically?" Do you mean physically or genetically?
China-ese: Chinese from PRC.
Taiwan-ese: Chinese from Taiwan.
Hongkong-ese: Chinese from Hong Kong.

Ethnically, yes they are all "Chinese", despite there are quite many differences between them in many aspects nowadays. Putting political discussions aside, the languages could be one of the major differences one could easily notice.

SPOKEN: Mandarin and Cantonese are two SPOKEN styles/dialects of Chinese language. Mandarin is the official state language of China and is the lingua franca of the country. It is in many areas the primary spoken language, including Beijing and Shanghai, although many provinces still retain their own local dialect. Mandarin is also the main dialect in Taiwan and Singapore. Cantonese is spoken by the people of Hong Kong, Macau and Guangdong province, including Guangzhou (previously Canton in English). Most foreign Chinese communities, such as those in London and San Francisco, also speak Cantonese thanks to emigration from Guangdong.

WRITTEN: There are Traditional Chinese & Simplified Chinese. Traditional Chinese is the writing system that Chinese-speakers have used for thousands of years. It’s a very COMPLICATED writing system – as you know, Chinese people doesn’t use letters the way Western languages do – we use characters that can stand for a variety of things. These characters have grown very subtle and complex over the years, and learning to write Chinese is a challenge even for Chinese natives. For foreigners, even trained translation services providers, it can be a very difficult process indeed. Simplified Chinese is exactly that – simplified. Promoted after World War II by the People’s Republic of China, the Simplified Chinese script originally was no different from Traditional Chinese in terms of vocabulary, just in the fact that most of the characters used to write out the language were literally simplified. Or we may say that Simplified Chinese is simply a simpler version of Traditional Chinese. The Simplified Chinese writing system differs in two ways from the Traditional writing system: (1) a reduction of the number of strokes per character and (2) the reduction of the number of characters in common use (two different characters are now written with the same character).

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