Explanation
Nested page table extensions, such as Intel Extended Page Table (EPT) or AMD Rapid Virtualization Indexing (RVI), are hardware features that facilitate the virtualization of memory. They allow the CPU to perform the translation of guest virtual addresses to host physical addresses in a single step, without the need for software-managed shadow page tables. This reduces the overhead and complexity of memory management for virtual machines, and improves their performance and isolation. Nested page table extensions do not directly affect the virtualization of other hardware components, such as network interfaces, host bus adapters, hard disks, or IO cache.
References:
* Second Level Address Translation - Wikipedia
* c - What is use of extended page table? - Stack Overflow
* Hypervisor From Scratch - Part 4: Address Translation Using Extended ...

Explanation
The output of xl list shows the state of the domains. The domain with ID 6, Ubuntu, has a state of "b-". This means that the domain is blocked, which means it is idle or waiting for I/O.
https://xenbits.xen.org/docs/unstable/man/xl.1.html