Texture is not showing in viewport always.
I want to adjust UV in UV editor by watching actual model in viewport, But I'm unable to View.

Did you connect the image texture node in the shader editor? also, I can't find your blend file, please provide it or explain the exact steps thank you.

It is not showing texture in viewport for every file and in every system in Blender 2.8.
Previously until Blender 2.79 It is working very well. We can able to see texture to object in viewport (until Blender 2.79) .

You have to make sure every needed setting are turned on.
You need to be in either look-dev or rendered mode, and the object should have a material with image texture node connected and an image loaded.
I have included a blend file take a look (open in blender-2.80).

There is no Texture mode in pie menu before and after applying texture

No, there is no texture mode in 2.80, you can use look-dev it gives almost the same result.

Then at least Look Dev mode should show texture.
But its not showing anything.
If the object won't show anything, How can We edit the Texture where to change?

In 2.79 if you opened image in UV/Image editor it was visible on a model in texture mode - but only in "Blender Render". You can not do that in 2.80. Texture needs to be connected to shader.

Thanks