Any difference between importing whole file and importing only class with show in Dart?

There is no impact on performance. The reason of using show is to reduce chances of confusion when importing classes from different packages.

For instance: Let's say

abc.dart has 2 classes

class One {}

class Two {}

And xyz.dart also has 2 classes:

class One {}

class Three {}

And you are importing both package in your file

import 'abc.dart';
import 'xyz.dart';

Say, you only want to use class One from abc.dart, so when you use One it could be from abc.dart or xyz.dart. So to prevent One coming from xyz.dart you'd use:

import `xyz.dart` show Three // which means only `Three` class can be used in your current file from xyz.dart package

When you use the keyword show basically what you are saying that I only want to use this specific class from this package in your dart file, from the docs:

Importing only part of a library

If you want to use only part of a library, you can selectively import the library. For example:

// Import only foo.
import 'package:lib1/lib1.dart' show foo;

// Import all names EXCEPT foo.
import 'package:lib2/lib2.dart' hide foo;

Now if you try to use anything from this package other than foo you will get an error because you specified that you only want to use foo.