How to swap UI of a custom rom with other rom? I have a custom rom which has Xperia like UI. It's very fast uses very less Ram. The other rom has Kitkat/Jellybean like UI but it has low performance as compared to Xperia UI like rom. 

I was wondering if I could change UI of Xperia rom with the other Kitkat rom. is it possible? 

I want to change Notification bar, menus , locker, Popup windows, buttons etc.

I use custom launcher and keyboard so no need to change that.
If your ROM has Theme Chooser integrated into the system, I would recommend just switching to an AOSP theme, like this KitKat theme.
You can check if you have Theme Chooser by doing any of the below:

Android 2.3: Check if there is an app in your launcher named 'Theme Chooser', with an icon of the Android robot colored four different colors
Android 4.0+: Check if there is any theme-related menus in Settings.

If your ROM does not have Theme Chooser, then you would have to get your hands dirty with manually themeing system APKs. Go to xda-developers for more info and tutorials on that.