In the below xml layout first I have created Framelayout then I have added RecyclerView and FloatingActionButton within Framelayout. You can see below program it will clearly describe you to add floating action button with RecyclerView xml layout in android.

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_margin="10dp"
    tools:context=".MainActivity">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recyle_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|right"
        android:layout_marginBottom="10dp"
        android:layout_marginRight="10dp"
        android:src="@mipmap/one"
        app:fabSize="normal" />

</FrameLayout>