CurrentVersion value in registry for each windows operating system

These are the required versions of windows operating systems:

Operating system              Version number
----------------------------  --------------
Windows 10                      10.0
Windows Server 2019             10.0
Windows Server 2016             10.0
Windows 8.1                     6.3
Windows Server 2012 R2          6.3
Windows 8                       6.2
Windows Server 2012             6.2
Windows 7                       6.1
Windows Server 2008 R2          6.1
Windows Server 2008             6.0
Windows Vista                   6.0
Windows Server 2003 R2          5.2
Windows Server 2003             5.2
Windows XP 64-Bit Edition       5.2
Windows XP                      5.1
Windows 2000                    5.0
Windows ME                      4.90
Windows 98                      4.10

If you want to know it by reading it from registry, you need to check the new CurrentMajor/MinorVersionNumber keys as well. The new values CurrentMajorVersionNumber (which is 10) and CurrentMinorVersionNumber (which is 0) under Windows 10. Those 2 keys are new in Windows 10 to detect Windows Version from Registry.

You can also use the following key for more specific string details about the OS

Reg query "\hostname\HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion" /v ProductName

I've found to be present for Windows XP and aboeve....

You should be using WinVer.nsh to check the Windows version...