Xen PV VM uses max 1 Thread - multithreading

Question

I am running a CPU benchmark tool (LINPACK) on a PV virtual machine inside Xen 4.5.1 on Ubuntu 15.10 x64 on an IBM x3550 M4 server. This tool should consume all possible CPU cycles available. I allocate 4 vCPUs by defining this in the Xen PV (test.cfg). However, LINPACK detects only 1 core and 4 threads while it should detect at least 4 cores:
CPU frequency: 2.494 GHz
Number of CPUs: 1
Number of cores: 1
Number of threads: 4
This is what lscpu says inside this Xen PV VM:
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 4
On-line CPU(s) list: 0-3
Thread(s) per core: 4
Core(s) per socket: 1
Socket(s): 1
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 62
Model name: Intel(R) Xeon(R) CPU E5-2609 v2 # 2.50GHz
Stepping: 4
CPU MHz: 2500.062
BogoMIPS: 5000.12
Hypervisor vendor: Xen
Virtualization type: full
L1d cache: 32K
L1i cache: 32K
L2 cache: 256K
L3 cache: 10240K
NUMA node0 CPU(s): 0-3
Other platforms such as Docker and HVM DO get cores allocated inside the virtual node (see below). These nodes have significant better performance then the Xen PV virtual node.
CPU frequency: 2.499 GHz
Number of CPUs: 2
Number of cores: 8
Number of threads: 4
This is the DomU Xen host machine lscpu:
lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 8
On-line CPU(s) list: 0-7
Thread(s) per core: 1
Core(s) per socket: 4
Socket(s): 2
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 62
Model name: Intel(R) Xeon(R) CPU E5-2609 v2 # 2.50GHz
Stepping: 4
CPU MHz: 2500.062
BogoMIPS: 5000.12
Hypervisor vendor: Xen
Virtualization type: none
L1d cache: 32K
L1i cache: 32K
L2 cache: 256K
L3 cache: 10240K
NUMA node0 CPU(s): 0-7
Xen vCPU list:
xl vcpu-list
Domain-0 0 0 7 -b- 10.1 all / all
Domain-0 0 1 2 -b- 6.5 all / all
Domain-0 0 2 4 r-- 2.6 all / all
Domain-0 0 3 0 r-- 3.9 all / all
Domain-0 0 4 3 -b- 4.4 all / all
Domain-0 0 5 6 -b- 2.6 all / all
Domain-0 0 6 5 -b- 4.7 all / all
Domain-0 0 7 7 -b- 2.9 all / all
test 3 0 1 -b- 1.5 0-3 / all
test 3 1 0 -b- 1.8 0-3 / all
test 3 2 0 -b- 0.7 0-3 / all
test 3 3 2 -b- 0.6 0-3 / all
xen DomU PV VM config:
cat test.cfg
bootloader = '/usr/lib/xen-4.5/bin/pygrub'
vcpus = '4'
memory = '2048'
cpus = "0-3"
Is there any options to give the paravirtualized guest the host-cpu topology? In other words, how do I get Xen to use more vCPU cores/ vCPUs?
Thanks!